And in an interview last month, Brad Carson, the under secretary for the Army, said the services were considering Purple Hearts for troops wounded while clearing improvised bombs made from old chemical weapons. This could result in honors for veterans of the Iraq war who were previously denied them.

That change, however, will cover a small number of veterans compared with the thousands presumably struck by American gunfire or ordnance in Southeast Asia, said Fred L. Borch, a retired Army colonel who is a historian of American medals and decorations and the author of “For Military Merit: Recipients of the Purple Heart.”

For troops wounded by their colleagues in Vietnam, recognition has been difficult to obtain.

Mr. Jones, now 70 and living in Arizona, twice asked the Marine Corps to issue the medals, including in 1981 with the help of Senator Barry Goldwater. He was told that victims of so-called friendly fire were excluded from the honor.

The policy effectively prevented ceremonies that would have acknowledged grave battlefield errors. It also compelled Mr. Jones to abandon his efforts. “I dropped it,” he said, “thinking there was pretty much no hope.”

For years he was plagued by sadness and guilt.

A change to the law in 1993 granted eligibility to those who had been wounded by American ordnance intended to cause an enemy harm. The law was amended, Mr. Borch said, after Congress became concerned that some victims of friendly fire in the 1991 Persian Gulf war had been awarded Purple Hearts but others had not. There had also been disagreement over the denial of the medal to an Army officer wounded in Panama in 1989.

“The genesis of the law was, ‘We want uniformity,’ ” Mr. Borch said.

Few Vietnam-era cases have been opened, veterans said, in part because the change was not widely publicized but also because the Pentagon has not actively searched for all of the victims. The complicated work of marshaling evidence has largely been left to veterans and their families or friends — many of whom, including Mr. Jones, were not aware the rules had changed.