It was a detail, perhaps carelessly overlooked: An old, beat-up portable radio, patched together with duct tape, that helped convince a D. C. Superior Court jury yesterday that two Washington men were guilty of murder.

The radio had been stolen from a Korean grocery in Southwest, where the owner of the store had been shot to death three years ago. The old radio might forever have gone unnoticed had not a twist of fortune intervened.

The slaying occurred on Dec. 13, 1979, when two men walked into the Fort Drum Market at 4686 Martin Luther King Ave. SW, tok $60 from the cash register and fatally shot grocer Boo Yeol Park, 50, in the chest.

In February 1980, detectives went to the home of Ulysses Jones Jr., 23, to arrest him in connection with the unrelated slaying of an off-duty Secret Service officer outside a bar. One of the detectives was Judy Roberts, who at the time also was working on the Park case.

Roberts had few leads in the Park case, but one of them was a report that an inexpensive plastic, Sanyo radio with a broken antenna and a section of tape that had been used to strap its batteries in place had been stolen from a customer in the grocery at the time of the slaying.

In the bedroom of Jones' home, Roberts spotted a battered Sanyo radio. When Jones was asked about the radio, police said, he replied that he had gotten it from a friend, Gregory Britt. Within a few weeks, Britt was identified as the second man involved in the Fort Drum Market case.

This week, jurors were told that Britt and Jones had known each other for years. According to prosecutor Harold Cushenberry, Jones, now 25, and Britt, 26, had gone to school together at Hine Junior High.

But except for the radio, which the customer identified, there was little evidence to place both men at the store on the day of the slaying.

Relatives of Britt said that he had been in Buffalo delivering candy and a card to a convalescing cousin. Defense attorney Robert Mosteller pointed out that several witnesses had failed to pick out Britt in police lineups.

There were no fingerprints, but there was an empty shotgun shell casing that matched a sawed-off shotgun that had been found in Jones' father's truck a year after the slaying.

Yesterday, Britt and Jones sat at the defense table as the jurors, after only two hours of deliberation, filed into the courtroom. As the verdict was read, Britt, who frequently had turned and smiled at spectators during the trial, sat expressionless. Jones, convicted in July of murdering Secret Service officer George I. Coaxum, cracked his knuckles and smiled.

Within moments, both were led away to cells by the federal marshals. Both men face possible sentences of life in prison. Sentencing was set for next month.