Some classified CIA documents and a letter apparently outlining an attempt at "internal blackmail" of the agency were found by a CIA employee in the underground garage of a Rosslyn office building in 1975, according to court testimony here today.

The testimony came at the espionage trial of Edwin Gibbons Moore II, a former CIA employee charged with trying to sell classified CIA documents to the Soviet Union in 1976. However, the prosecution made no connection today between More and the documents found Feb. 7, 1975, in the garage of the Ames Building, rented mostly by the CIA.

Testimony indicated that those documents - "20 or 30 sheets of paper," tied as secret, were found at 5:10 p.m. by a CIA accountant, Donald L. Roberts, as he was walking to his car. Included among the papers, witnesses testified, was a letter addressed to William E. Colby, then director of the CIA.

The letter said that "old-timers anti careerists" at the CIA were unhappy about the fact that they had not been recently promoted and urged Colby to promote them soon, even if they already had retired from the agency, according to testimony.

Former CIA security director Charles W. Kane read from the letter, "I realize this is internal blackmail . . . " then was cut short by objections from defense attorney Courtland K. Townsend Jr.

The letter and the documents found in the garage were sealed on grounds of national security by U.S. District Judge Frank A. Kaufman and were not available to the public in court records.

Prosecutors declined today to say whether anyone had ever been charged in connection with the 1975 incident.

Other documents mentioned so far at Moore's trial here include a classified CIA telephone directory, pages of which were cut up in the manner of a "jigsaw puzzle," and pages with "illegible handwriting," several FBI agents testified. These documents area also sealed from public view.

Moore, 56, of 4800 Fort Sumner Dr., Bethesda, has been charged with one count of espionage stemming from his alleged attempt to sell classified CIA documents to the Soviet Union for $200,000, on Dec. 21, 1976. He also has been charged with four counts of unauthorized possession of classified document property.

Moore, who worked for the CIA for 22 years before retiring in 1973 as a GS-9 office worker, has pleaded innocent by reason of insanity.

FBI agents who entered Moore's $100,000 Bethesda home with a search warrant charged with espionage today gave these descriptions of some of the thousands of pages later taken away in eight cardboard cartons:

Among the seized items, were several apparent preliminary drafts of the letter Moore allegedly included with the packet of CIA documents found inside the gates of a Soviet residence at 3875 Tunlaw Rd. NW, Washington. One of the apparent preliminary drafts was entitled, "Operation Respect and Recognition." In offered, a "once in a little opportunity" for someone to buy "a gold mine" of CIA information for "the very reasonable price of $10 million."

The draft offered what it called "a new twist in espionage," in which classified documents were to be exchanged before money was received. Payments were to be made in old bills, "and possibly a few bars of gold."

Another apparent preliminary draft said reinuneration might be sent to the writer "through your diplomatic pouch."

Stating that "I shall be in complete control," the writing on this draft said that "all you want is information, and all I want is money." This draft also made reference to Phillip Agee, a former CIA agent who had earned money from a book he wrote about the agency.

Prosecutor Thomas L. Crowe asked several FBI agents to describe how classified documents they seized at Moore's home and originally dated April 4, 1972, and May 23, 1972, had been altered to make it appear as if they had been written on those dates in 1975. The agents said the technique involved use of white ink to eliminate the original date, the typing of the new date over that, and then the photocopying of the material. The photocopying procedure "equalized" the difference in looks between the original and new types, the agent said.