U.S. District Judge Barrington D. Parker said yesterday that he would allow a convicted submachine gun salesman to testify about part of his jailhouse conversations with one of the Cubans charged with the 1976 bombing assassination of former Chilean ambassador Orlando Letelier.

Parker ruled that Antonio Polytarides can tell the seven-women, five-man jury today about a late 1978 conversation he had with Guillermo Novo Sampol in which Polytarides said that Novo said he "was very angry."

Novo, a leader of the staunchly anti-Castro Cuban Nationalist movement, claimed in the conversation that members of the Cuban exile group had "been betrayed by certain people in the (Letelier) case, but they will pay them back," Polytarides has testified with the jury sequestered outside the courtroom.

However, the judge ruled that Polytarides cannot tell the jury about an earlier conversation in which Novo allegedly admitted that "our group," meaning the Cuban Nationalists, "was responsible" for the Letelier assassination. Parker said that particular conversation was initiated by Polytarides, which was not the case with the late 1978 dialogue.

Parker put off until today a ruling on whether convicted extortionist Sherman Kaminsky could testify about conversations he had with Alvin Ross Diaz, another Cuban exile charged with the Sept. 21, 1976, slaying of Letelier.

Polytarides and possibly Kaminsky are the last government witnesses against Novo, Ross and the third defendant in the case, Novo's brother Ignacio. The defense told Parker yesterday that it would call Letelier's widow Isable as its first witness. She previously testified for the prosecution, but the defense wants to question her about the contents of Letelier's brief-case found in the wreckage of his car after it was bombed along Washington's embassy row.