High above the Model-T in the lobby, the rat-a-tat noise on the fourth floor of the Museum of History and Technology quietly came to a stop Monday when the Smithsonian Institution switched its switchboard.

The old switchcoard, a 701 Step-by-Step, plug-it-in, pull it out, model featured in many a 1930s movie, was replaced by Bell Systems' computerized Centrex II -- a complex state of the art system that promises to do everything but sweep the floor.

The conversion, Smithsonian officials said, will have little effect on the public. New numbers will replace old ones and a 4.5 second wait to connect will be cut in half. The most impact will be on the seven women who staff the machine -- the speediest wrists in the East.

Ninety-nine pages of Smithsonian telephone numbers -- 2,200 numbers the operators have memorized -- will be changed.

Suprisingly, none of the operators are upset about the change.

"Oh, it will probably blow our minds for a few days," supervisor Pat Tilko, a 22-year veteran predicted, "But after that, we'll be as quick as ever.

"There's just something magical," Tilko explain," about sitting in front of the switchboard. Numbers flash in front of my eyes that I wouldn't be able to remember two steps away from the board."

Under the old system, five operators sat side-by-side on vinyl girded high stools waiting for one of the 12,000 daily calls to flash across the board. When a light flickered, elbows would fly as swift-moving hands reached for the two snake-like cords needed to connect the call, plugging one cord into one of the 1,700 slots and the other cord into another. Two microphone keys wer flipped and the convention was made.

Under the new system, each operator will have a separate desk with a touch-dail console.

"Our job," said Margaret Frank, an operator who served under the command of Gen. Doolittle in England during World War II, "will undoubtedly be more relaxed. We will be doing less work."

All transfer calls, representing some 3,000 operator-assisted calls a day, between Smithsonian offices should be connectable without the aid off a switchboard. And employes will also be able to hook up independently with the computer terminal and dial their own long distance calls.

In addition to alleviating the operators' work load, Smithsonian officials say the new system opens up more telephone lines and frees the several rooms which house the Step-by-Step for much needed office space. The bulk of the Centrex II equipment is at the C & P facilities in Washington.

Smithsonian communications chief John Moreci said the Step-by-Step had reached its capcity and could not process any new telephone mumbers. The new system, which costs the same to operate as the Step-by-Step -- $70,000 a month -- "has an almost infinite ability to expand," he said.

Although the old system had become an anachronism in terms of the Smithsonian's needs. Moreci said, it will not be displayed with the other remnants of Americana at the museum.

Most likely, he said, comparing the difference between the two phone systems to a vintage auto-mobile and a modern luxury car, the Step-to-Step will continue to flicker in a foreign home where less advanced technology is required.

"The Model-T is dependable, reasonably priced and performs the same basic functions as a 1980 Cadillac -- it eventually gets you there," Moreci said. But, he added, "there are just some things the new equipment can do that the old cannot."