In a surprise move yesterday, federal prosecutors dropped a charge of assault on a police officer against Washington school board member Frank Shaffer-Corona.

It was made clear why the government took the action in D.C. Supreme court. Shaffer-Corona was arrested on June 7 following a scuffle with police in a small restaurant on Columbia Road NW.

U.S. Attorney Charles Ruff would say only that the case against Shaffer-Corona was dropped because "our investigation developed insufficient evidence to justify prosecution."

According to court records, police officers were serving a fugitive arrest warrant on a man they thought was Henry Salmen, who is wanted in West Virginia for jumping $2,000 bail in a burglary case. But they arrested the wrong man.

The man involved is Manuel Gascon, 25, of 3350 17th St. NW, who is employed as a painter, according to court records.

Detective M. A. Tuarneir of the D.C. Police Department's fugitive squad sent a note to the U.S. attorney following the arrest, requesting that the case against Gascon be dismissed because of the mistaken identity.

The case against Gascon was dropped June 13.

According to courthouse sources, the fact that Gascon turned out to be the wrong man was not in itself grounds enough to dismiss charges against Shaffer-Corona. But the mixup may have greatly weakened the government's case, the sources said.

Saffer-Corona was arrested at a restaurant at 18th Street and Columbia Road NW about 3 a.m. He said he was having coffee when several police officers entered the restaurant and made the arrest of the suspected fugitive. Police said that Shaffer-Corona interferred with them and struck one of the officers.

Shaffer-Corona was ordered released from jail shortly after his arrest by D.C. Superior Court Judge William S. Thompson, who had been telephoned by School Board Vice President Barbara Lett Simmons.

After charges against him were dropped yesterday, Shaffer-Corona said he would sue the city for false arrest, brutality and defamation of character.