A nationally known examiner of "questioned documents" testified in federal court today that a key 1967 memo supporting police in the Prince George's County "death squad" trial here "is as old as it purports to be and was not fabricated in 1979 or at a later date."

The testimony by Lyndel L. Shaneyfelt came after attorneys for plaintiffs suing police in the $9 million civil rights case expressed surprise in questioning assistant police chief Joseph D. Vasco earlier this month when Vasco said he found the 15-year-old memo in his home after The Washington Post published the so-called "death squad" stories in 1979.

In the allegations, police are accused of setting up a series of convenience store robberies in 1967 in which two men were shot and killed by waiting police, and several others were arrested.

Families of the two slain men and two other men arrested in the incidents sued Vasco and other police in 1980 after The Post articles were published.

The memo that Vasco said he found in his home contradicts allegations that he and other officers instructed informers to recruit participants for the robberies. Dated June 6, 1967, the memo is a typed report composed by Vasco in which informer Gregory Gibson tells him that two acquaintances want him to join them in a robbery. Police subsequently staked out the targeted store and killed one suspect, William H. Matthews, and arrested a second, Marvin Rozier. Police said Matthews ignored orders to halt and fired his pistol at officers before or at the same time that one of the officers fired at Matthews.

Today, Shaneyfelt testified that the Vasco memo--a carbon copy of the original--showed signs of "extensive wear . . . and dirt" that was consistent with a 15-year-old document and would be difficult to fabricate in 1979 or later. Vasco said he has been unable to find the original of the memo.

Shaneyfelt, a retired FBI document examiner who testified before the Warren Commission on the assassination of President John F. Kennedy in 1963 and on the authenticity of billionaire recluse Howard Hughes' will in 1976, also said that characteristics of some of the letters typed on the Vasco memo were similar to those of other 1967-era police reports he used for comparison.