It was one of the coldest cases in the Los Angeles County Sheriff's Department's cold case file, the unsolved murder of two young police officers here after a traffic stop early in the morning of July 22, 1957.

No major break in the investigation had occurred since 1960, when a Manhattan Beach homeowner found two watches and a gun in his backyard and gave them to the police. The watches had been stolen from two teenagers assaulted in the area on the night of the killings. The gun was traced to a Sears in Shreveport, La., but the trail went dead there.

The case remained open but inactive for four decades, until, the police said, a tipster called detectives last September to identify the killer. The tip proved false, but as a result of reopening the case, the police decided to check fingerprints they had had on file since 1957 against a nationwide computerized database set up by the F.B.I. last February.

The prints led to Columbia, S.C., where early this morning the police arrested Gerald F. Mason, a 68-year-old retired gas station owner living in a comfortable suburban tract northwest of town. He is being held without bond pending an extradition hearing.

The police have found no record of any crimes committed by Mr. Mason since 1957; there was only one burglary charge in South Carolina from 1956. It was the fingerprints from that arrest that appear to match prints taken from the stolen car the presumed killer was driving the night the two El Segundo officers were shot.

The police put Mr. Mason under surveillance weeks ago, and he apparently was unaware that he was being watched. He played golf on Tuesday and was arrested at his home at 7 a.m. today.

Chris Mills, Mr. Mason's lawyer, said California had begun extradition proceedings. He said Mr. Mason was arrested on a fugitive warrant by a contingent of United States marshalls and South Carolina and California law enforcement officers.

''The family is in shock,'' Mr. Mills said. ''We're talking about a man who has led a law-abiding life here for 42 years or more.''

Mr. Mason's neighbor of 10 years, Betty G. Wiggins, said: ''What gets me is why would it take so many years to find somebody who has been so well known here in Columbia? It's not like he was living like a fugitive, hiding or running away from something. That's why I just know they've got the wrong man.''

Ms. Wiggins said Mr. Mason and his wife, Betty, have two daughters and three grandchildren.

Relatives gathered at the home today. Mr. Mason's younger brother, Murray, said he and the family believe the arrest is ''a case of mistaken identity.''

''If he's ever even been in California, we don't know when it could have been,'' he said.

The 1957 case of the Lover's Lane Bandit has haunted and frustrated the El Segundo police, the Los Angeles Sheriff's Department and the Los Angeles County District Attorney's office. Law enforcement officials reserve a special contempt for police killers, particularly the ones who elude capture for years.

Around midnight on July 21, 1957, a man accosted two teenage couples necking in their cars along a well-known lover's lane on Van Ness Avenue in Hawthorne, just east of El Segundo. He tied up the four teenagers, robbed them, forced them to strip and raped a 15-year-old girl. He stole their 1949 Ford and drove off into the night, with the girls' skirts lying on the floor of the back seat.

About an hour and a half later, Officers Richard A. Phillips, 28, and Milton G. Curtis, 25, of the El Segundo police saw a car run a red light at Rosecrans Avenue and Sepulveda Boulevard, a quiet, undeveloped area of tall eucalyptus trees near a Standard Oil tank farm. The officers ordered the driver out of the car. Just then, a second El Segundo cruiser pulled up. The arresting officers waved the car on, thinking they had the situation under control.

The driver then pulled out a .22-caliber snub-nosed revolver and shot both officers. Officer Phillips fired several shots at the fleeing car and radioed for help before losing consciousness. Both officers died before reaching the hospital. Officer Phillips had two years on the job, Officer Curtis two months.

The driver abandoned the car four blocks away and ran south into Manhattan Beach through yards and over fences, dropping the watches he had stolen from the teenagers, the police said, and his weapon.

Despite a wide manhunt involving local, state and federal police, no further trace of the killer was found. The case was featured in True Detective magazine in 1958 with a plea for public help to solve the murders.

''We followed literally thousands of leads and tips, but with no success,'' said Jack Wayt, chief of the El Segundo police. ''But we certainly never forgot this case. The officers are long gone, but they never were forgotten.''

There was a flurry of activity three years after the shootings, when the watches and the gun were recovered, but it led nowhere. The case drifted into the inactive file.

Early last year, the F.B.I. established a database that for the first time compiled fingerprints from police agencies all over the country. Law enforcement agencies began looking at old cases to see whether this new tool could help. The Los Angeles District Attorney's office reopened 3,000 unsolved homicide cases dating to 1980 with the aid of the new database.

But it did not look back 45 years, until the El Segundo police received the tip in September.

''That's the way these things go sometimes -- a fluke, a tip,'' said Capt. Frank Merriman of the Los Angeles County Sheriff Department's homicide bureau, who oversaw the handling of the case. ''It turned out to be wrong this time, but it caused us to look in the right direction.''

The police had several good prints from the stolen car, which matched the prints from the 1956 South Carolina burglary arrest. No good prints were on the gun for comparison, because it had been in the ground for three years, Mr. Merriman said. He added that ballistics tests from the weapon were ''consistent'' with the bullets that killed the officers, but were not conclusive because of the poor condition of the gun.

The authorities said one of the reasons it took so long to find Mr. Mason, a factor that may complicate any prosecution, was that he had led a spotless life since 1957. They said it is unusual for someone to commit a brutal crime just once.

In the county's request for extradition, Mr. Mason is charged with two counts of murder and multiple counts of kidnapping, robbery and rape. There is no statute of limitations on murder, and the other charges can be brought because Mr. Mason had left California within three years of the crimes, which suspends the state statute of limitations.

Steve Cooley, the Los Angeles County District Attorney, said that he was barred from seeking the death penalty because the California death penalty law on the books in 1957 was later invalidated by the United States Supreme Court.

John Booterbaugh, 79, served on the El Segundo force with the two officers and said today that he was shocked that someone had been charged in their killings.

''I thought it would never, never happen,'' Mr. Booterbaugh, said outside the office of the El Segundo police department, from which he retired in 1971. ''The tragic part about the whole thing is that some of the old timers I worked with are no longer around here to see this.''