THEY can fix anything here at the nation's most exclusive repair shop, housed in a huge ramshackle building that was once the elementary school of Harpers Ferry, W. Va. "They" are the people at the National Park Service's Conservation Laboratories. With just one client, the United States government, they work only on national treasures -- objects normally found in the White House or in the 300 areas urn by the Park Service. At any one time, they handle or store about $3 million worth of objects -- 6,000 to 8,000 of them -- most of which arrive in wretched shape. In past years, such divers items as the tent Geogre Washington used at Valley Forge, Robert E. Lee's red velvet sofa and Jimmy Carter's desk from the Oval Office have come here for repair.

Incoming objects are carefully listed, numbered, labeled, fixed and logged out. "In the six years we've been operating, we've never lost anything," says Arthur Allen, chief of the division of museum services, which runs the labs. The work is done in seven specialized labs, headed by conservation experts.

The conservators bring long years of scholarship and experience to their tasks. For example, textile expert Toby Raphael has a master's degree in museum studies from George Washington University and further training at the Smithsonian and a conservation center in Mexico City. Janet Stone, a naturalist, studied at the Cooperstown, N. Y., Conservation Graduate Center and was curator of the Sierra Leone Museum.

In the paper lab, documents, sketches and etchings are restored. In the ethnology lab, experts repair textiles, leather and beadwork, including a rich collection of Indian artifacts.The fine metals lab has the nation's largest gun collection.Other labs handle archeological finds, glass and ceramics, paintings, wood and furniture.

In 1970, the Park Service moved the people who prepare its movies, publications and exhipbits to this quaint town's National Historical Park to work in a creatvi, quiet setting away, but not too far away, from Washington. The exhibits experts soon found that many of the historic artifacts needed professional care before going on display. So in 1973, the division of museum services was created. "The demand for our work quickly went beyond display objects," says Art Allen. Now the division's 13 employes plus a handful of students and volunteers take on repairs for the entire Park Service. But their pleasure and privilege is to work on the kinds of projects pictured here.

GET THAT TO THE GUNNERY -- When a shotgun from the estate of William Floyd arrived, lab experts here winced. "It was a basket case," one recalled.

The care it had received at the home of the signer of the Declaration of Independence was so slipshod that the gun's steel barrel had rusted to a dull red. Its walnut stock was infested with powder post beetles. Conservator Bart Rogers and assistant Charles Shepherd, shown with the gun, cleaned the steel electrically in a sodium carbonate solution. Then they treated the wood with pentachlorophenol to kill any bugs. Finally, the bug holes were filled with a wax mixed with pigments to look like wood. The gun was fixed, not to fire again, but to be put on exhibition at Floyd's estate on Fire Island, N.Y. The repair took the two men 30 hours.

LONGER LIFE FOR LINCOLNALIA -- Abraham Lincoln's furniture from his home in Springfield, Ill., came here a year ago in terrible condition. Chair backs were held together with wires, table legs were rotted and veneer was missing from several pieces.

This furniture -- six Empire-style dinning-room chairs, a cupboard, a chest of drawers, a small pedestal table, marble-topped table, framed mirror and cushion -- was bought by the Park Service from the Pennsylvania Historical Society in Philadelphia more than a year ago.

When the pieces arrived here, they were carefully photographed, as a documentary record, a procedure the labs do with each piece they receive. The horsehair seats of the dinging-room chairs were in shreds and beyond repair.Lab experts had to order new horsehair from an upholstery supply firm in Baltimore; and then the original horsehair was put into plastic bags, to be returned with the refurbished chairs. 'We never throw anything out," says conservation chief Arthur Allen. "We let the local museum curators decide how much original material to keep for documentary purposes."

Repairing the chairs was a breeze compared with the task of restoring Lincoln's bureau. Three hundred pieces were missing from its mahogany veneer. "We found patches of old wood to match the grain in about 100 places," explained furniture conservator Allen Cochran. "But in the other 200 places the gaps were so tiny we couldn't cut wood for them; so we used wood fillers and stained them to match the surrounding wood. At first glance you wouldn't notice the work, but under a microscope it looks like a wooden patchwork quilt."

The marble-topped table was broken into serveal pieces. Ceramic experts glued them together and when they through, found they still had a two-inch triangular gap. They constructed a new marblelike piece using a clear polyester resin and marble dust. The tabletop now looks solid; only a very close look will show the breaks. The restored furniture went back to Springfield in a heated van last month.

Tricky furniture repair is not new to Cochran. Before joining the National Park Service in 1972, he was a cabinetmaker and furniture and upholstery restorer in the Winchester-Berryville area of Virginia. He ran his own business there for more than 20 years. "I had plenty of difficult jobs when I was on my own," he recalls. "But now all of them are difficult."

CONCRETE MANSION FOR THE MASSES -- Thomas A. Edison, better known for inventing the first practical light bulb,the phonograph and the movie camera, devised this model house in the early 1900s to promote his cement business. He claimed the house could be built "by machinery in lots of 100 or more at one location for a price which will be so low that it can be purchased or rented by families whose total income is not more than $550 per annum." The seven-room concrete house could be cast in six hours at the cost of $1,200 where sand and gravel were easily available, nd labor, he said, would cost $150.

Describing his proposal, Edison declared, "I think the age of concrete has started. Then even the poorest man among us will be enabled to own a home of his own -- a home that will last for centuries with no cost for insurance or repairs, and be as exchangeable for other property as a United States bond."

Edison's model house came to the furniture lab here with missing railing, doors, windows and pillars. Restorers made everything from glass windows to porch clumns.Conservator Allen Cochran, shown replacing a pillar on the house, made rubber molds for the missing pieces. Catings were made with a material like plaster of Pairs, and painted to resemble the originals.

In fact, edison did have a chance to see some of his houses mass-produced. A project was begun in 1919 for them in Union, N.J., and the CONCRETE WAS POURED IN ONE DAY. His mass-production scheme never went national, but the medel he designed is exhibited in his lab in West Orange, N.J., as another relic of his fertile mind.

THE BUNTING THAT TRIPPED JOHN WILKES BOOTH -- The U.S. Treasury Guard flag used bunting around Abraham Lincoln's box at Ford's Theatre the night he was killed has its own history.It belonged to the Treasury Department's militia, whose function was to guard the president -- and the department -- in the event that the Confederates got across the river and into Washington. The Lincolns had decided at the last moment to attend the showing of Our American Cousin at the theater the night of April 14, 1865, and theater officials, after a frantic search for something appropriate, grabbed the flag to drape the box.

After shooting Lincoln, john Wilkes Booth leaped from the box to the stage and shouted, "Sic semper tyrannis! The South is avenged!" But he caught his spur in the folds of the flag bunting, ripping a four-inch tear in it, and broke his left leg. He managed to limp across the stage and down the stairs to the rear of the theater where a horse was waiting.

He rode off into the night but died 12 days later -- either by suicide or a soldier's bullet -- as federal agents and Army officers set fire to the barn where he was hiding.

The blue flag with a painted eagle emblem in the center that had decorated the president's box that night was put on display at Ford's Thatre when it recopened. Over the years the flag's silk became deformed by fluctuations in the temperature and humidity, it color faded and its emblem began to crack and split.

The problems were so serious when the flag was brought to the Conservation Laboratories last year that a fabric specialist from Bern Switzerland, was flown in. For a month the expert, Marie Masson, and Park Service conservator Toby Raphael worked to restore the flag. They removed its damaged backing, vacuumed it, washed it in mineral-free water with a neutral detergent, dired it rapidly under glass so that the paint would adhere better and replaced its backing with three layers of flannel. For a frame they wanted low-acid wood, so they made one of birch and bassweed.

Now restored, the flag -- with its four-inch tear carefully preserved (the small white streak on the right side of the flag in the picture) -- was shipped back to Fordhs Theatre in January with the advice that it be displayed in a climate controlled room of 55 percent humidity and 60 to 65 degree temperature.