About 9 a.m. on Monday, May 12, 1975, Joel Hersh, a guard for Dunbar Armored Express, Inc., walked into the U.S. Department of Transportation building at 6th and E Streets SW, carrying three bags containing $49,464.67.

As he stepped into an elevator, a man behind him snatched Hersh's gun from its holster. Hersh raised his hands and the moneybags fell to the floor. The other man picked them up and fled.

The gunman bumped into another DOT employee and dropped one bag containing $9,400 as he ran toward a waiting getaway car on E Street. He got into the car and that was the end of the robbery.

Prosection and defense attorneys agreed on this sequence of events during the trial in U.S. District Court this week of Eugene P. Patterson, 29, on charges of armed robbery and interference with commerce by robbery, a federal charge, growing out of the holdup at DOT.

The question for the jury was whether Patterson did it.

According to federal records, Patterson, formerly a resident of Takoma Park, was deported from Canada last November after serving 90 days for illegally disposing of the body of his wife in the back yard of the house he occupied in Montreal. Canadian officials were unable to determine the cause of the woman's death because the body was decomposed, officials here said.

Patterson was taken into custody by federal authorities in Champlain, N.Y., after he crossed the border. He was returned to Washington to stand trial for the robbery at DOT. How he came to be charged in the DOT case is a tale involves a tip to the FBI, telephone records, and a sales contract with a suburban Washington automobile dealership, according to evidence at the trial and officials who are familiar withe the investigation.

The tipseter reported seeing Patterson dividing up a large sum of money with another person. Authorities subsequently learned he was in Canada through records of telephone calls he made to his family in this country.

At his trial, Patterson testified that he was in Philadelphia visiting his mother and a sister at the time of the holdup.

But Assistant U.S. Attorney Mary Weiss introduced into evidence a contract Patterson had signed with a suburban car dealership to purchase a van. The contract was dated May 12, 1975, and stamped with the time it was signed - 10:30 a.m.

Weiss maintainee that this showed Patterson was in the Washington area the day of the holdup, that the crime had produced the money to buy the van, and that he had time to go from DOT to the auto dealership and sign the contract by 10:30 that morning.

The jury deliberated for almost four hours yesterday before returning a verdict of guilty. Patterson faces a minimum of five years in prison and a maximum of life. Judge George L. Hart Jr. set no date for sentencing.