In dramatic testimony, Michael V. Townley, the confessed assassin of Orlando Letelier, said yesterday that one of the Cuban exiles charged with that murder "did nothing directly" to help him carry out the killing.

Townley, an American-born, Chilean secret police agent who is the government's key witness against three anti-Castro Cubans charged in connection with the Letelier killing, gave the surprising testimony about Alvin Ross Diaz, who is charged with conspiring to murder Letelier and with other offenses in the case.

Under a stiff cross-examination from Lawrence Dubin, Ross' defense lawyer, Townley also said yesterday that he gave what he described as an "incorrect answer" to a question put to him by U.S. District Court Judge Barrington D. Parker last August when Townley pleaded guilty to conspiring to murder a foreign official.

Parker had asked Townley whether he had had any experience prior to the Sept. 21, 1976, Letelier killing with the use of radio-controlled bombs such as that used in the Letelier assassination. Townley replied he had not had such experience, a statement he said yesterday he didn't know why he made. As early as 1975, Townley has said he built a similar bomb for an aborted Mexican assassination mission.

Parker was clearly annoyed by the change in Townley's testimony. Townley, 36, fidgeting in the witness chair, looked at Parker and said, "I humbly apologize for making that answer. I might have been confused at that time."

"You didn't indicate you were confused then," Parker told Townley.

"No, I did not," Townley said.

Townley's testimony about Ross and his experience with radio-controlled bombs came on the fourth day of cross-examination of him by defense lawyers Dubin and Paul Goldberger. During that time, Townley said he repeatedly lied under oath last year to a Chilean investigator assigned to try to find out what role the Chilean government had in the killing of Letelier as he drove along Washington's Embassy Row. Letelier, former Chilean ambassador to the United States during the Marxist government of Salvador Allende, became in exile an outspoken critic of the military junta of Augusto Pinochet.

But Townley said his lies were part of a cover-up of the Chilean secret police's involvement in the killing that was directed by Juan Manuel Contreras Sepulveda, the former head of the secret police, once known as DINA. Contreras is awaiting a Chilean Supreme Court decision on whether he is to be extradited to stand trial in the Letelier case.

"You considered it more important to follow the orders of Gen. Contreras than the oath you took to tell the truth," Dubin asked Townley at one point yesterday.

"Yes," Townley replied.

Townley has testified that it was his "recollection" that Ross attended a Sept. 13, 1976 meeting at the Chateau Renaissance Motel in Union City, N.J., at which Townley asked members of an anti-Castro group known as the Cuban Nationalist Movement to help in the assassination of Letelier.

But Townley said yesterday that when he testified last May before a federal grand jury that was investigating the Letelier assassination, he named three persons at the meeting -- none of whom was Ross -- and had "no memory" of others who attended. The three he named sere Guillermo Novo Sampol, a Cuban exile charged with the Letelier killing and now on trial, and Jose Dioniso Suarez Esquivel, a fugitive indicted in the case, and the late Dr. Juan Pulido.

Asked later to name "one thing that Alvin Ross did to help you kill Orlando Letelier," Townley responded:

"Mr. Ross did nothing directly to help me kill Orlando. I have no idea what aid, if any, he gave to any other persons. To myself directly, none that I know of."

"From the time you left New Jersey on the 16th or 17th or 15th (of September 1976) to go to Washington to plant the bomb until you actually planted the bomb, you had no contact with Alvin Ross, did you?" Dubin asked.

"No sir," Townley answered.

The indictment in the case, however, charges that Ross met with Townley twice in New Jersey after the bomb was planted in Letelier's car and that they discussed Townley's mission in Washington.