A former member of the Navy's elite hostage rescue team, Seal Team Six, has pleaded guilty to filing false travel claims totaling nearly $13,000 in 1983 and 1984 and has agreed to cooperate in a financial investigation of the secret unit.

John Mason, 33, of Phoenix, a one-time Seal Team Six medic and weapons specialist who took part in the U.S. invasion of Grenada in October 1983, entered the plea in U.S. district court in Norfolk Tuesday. Sentencing is scheduled Nov. 12; Mason faces up to five years in prison on each of the four counts to which he pleaded guilty.

Joseph J. Aronica, an assistant U.S. attorney in Alexandria, said the government was prepared to prove that Mason submitted false receipts for lodging aboard a cabin cruiser docked at Washington's waterfront, and for training courses that did not exist, including one called "The Explosive Entry Development Course."

Two other members of the unit have pleaded guilty in courts-martial to stealing thousands of dollars by filing phony purchase receipts. Navy officials have said the Naval Investigative Service is examining "a roomful" of documents dealing with millions of dollars in expenditures by the team since it was formed in 1980.

Although its mission is classified, the team is known to be the Navy's equivalent of the Army's Delta Force hostage-rescue team.

Seal Team Six members took part in the U.S. invasion of Grenada in 1983, and were sent to the Middle East during the 1985 Achille Lauro cruise ship hijacking, and to aid in the unsuccessful search for kidnaped Beirut Central Intelligence Agency station chief William Buckley, who was later killed.

Prosecutor Aronica said in court that in February 1984 Mason claimed $3,253.49 in expenses for a trip to Austria and Germany, including $1,524 for a nonexistent course fee.

He said Mason adapted the phony receipts from a packing list of arms sent to the team by the Hechler & Koch arms makers.