U.S. District Judge Barrington D. Parker yesterday postponed any acceptance of a guilty plea from Michael Vernon Townley, the key government witness in the Orlando Letelier murder case.

Parker did not disclose his reasons for saying he wanted to review the plea-bargaining agreement before deciding whether to accept it. But The Washington Post learned later that the snag developed over a section of the agreement in which the government agreed that a specific prison term would be imposed upon Townley.

The presentation by prosecutors of a plea-bargaining agreement involving a specific prison-term promise for a defendant is extremely rare here, since U.S. judges prefer to exercise sole authority over the sentencing process. However, such agreements are legally allowable and are used regularly in some jurisdications.

The exact length of the prison term to which government prosecutors agreed for Townley could not be ascertained, although it reported would require him to serve "substantial" time in prison. The count to which Townley has agreed to plead carries a maximum prison sentence of life.

Townley, who grew up in Chile and became a secret police agent there during the current Chilean military regime of Gen. Augusto Pinochet, has agreed with prosecutors that he will plead guilty to one count of conspiracy to murder Letelier, former Chilean ambassador to the U.S.

Letelier and a coworker, Ronni K. Moffitt, were killed Sept. 21, 1976, when a bomb exploded under the car in which they were driving around Sheridan Circle NW.

According to court testimony by an FBI agent, Townley has admitted that he was sent by the secret police agency, DINA, to the U.S. to assassinate Letelier, and actually placed the bomb under Letelier's car.

Townley's agreement three months ago to plead guilty in the case is believed to have been a major break leading to Tuesday's indictment of five Cuban exiles in the U.S. and three DINA officials in Chile for Letelier's murder.

Townley served as the conduit between the two groups as well as being directly involved in the actual assassination, according to investigators.

The still-secret plea-bargaining pact, in which Townley reportedly agrees to provide information only about activities he many have been involved in on U.S. soil or involving U.S. citizens or property, was worked out in more than two weeks of negotiations between his attorney, Seymour Glanzer, and the government.

Reporters had been notified yesterday, in the usual Justice Department manner, that Townley's plea was expected before Judge Parker at 2 p.m. The proceeding was delayed, however, by a meeting in the judge's chambers involving prosecutors and Glanzer.

Around 2:40 p.m., Townley - a beaded, thin man wearing a blue suit - was brought courtroom to join the attorneys who had assembed there.

Where Parker tood the bench, he called Townley forward and told him that he had just learned about the plea-bargaining agreement and that he did not feel the court had been "fully advised" yet about the deal.

Parker said he wanted to "step back and review" the agreement, and reschedule the plea for some time in the fear future.

"As a result, there will be no proceeding at his point," Parker said.

The indictment accuses former DINA head Gen. Manuel Contreras Sepulveda of ordering Letelier's murder, and two other DINA operatives in Chile with executing the details of the plan.