By JAY DUCASSI

Guillermo Novo Sampol, an anti-Castro militant jailed in connection with the 1976 assassination of former Chilean diplomat Orlando Letelier, has been released from prison again. This time apparently for good.

Novo, 41, who was released by mistake last October from Danbury Federal Correctional Institute in Danbury, Conn., and returned to jail voluntarily when the error was realized, was freed last Monday, prison authorities said Thursday.

He was "mandatorially released," said prison spokesman Scott Miller. He explained that this meant that Novo had completely served the 18-month sentence passed last June 26.

Miller, however, did not explain how the sentence could have been fully served when Novo still had about a year to go in his term.

Miller said that the release was not a parole release, something Novo had been seeking since his return to prison.

In fact, after he returned to the jail and went before a parole board it denied his request for freedom and ordered him to serve the full sentence.

Neither Novo nor his lawyer could be reached for comment.

Novo, his brother Ignacio, and another man, Alvin Ross Diaz, were convicted and later acquitted in relation to the Letelier case.

Letelier, who had been Chile's ambassador to the United States under the Socialist regime of Salvador Allende, was killed while in exile in the United States in 1976, when a bomb exploded under his car in Washington, D.C.