By NERY YNCLAN

Guillermo Novo Sampol thought the parole board might give him his freedom last week when he voluntarily returned to a Connecticut prison that released him by mistake.

What he got was another year in prison.

Novo "was hoping the whole mistake would help him out at the [parole board] hearing," his mother said from New York. "As soon as he got the message [that he had been freed by mistake] he, in good faith, returned to Connecticut."

Novo, an anti-Castro militant serving an 18-month sentence for lying to a grand jury in connection with the 1976 murder of Chilean diplomat Orlando Letelier, informed his lawyers and mother that the parole board recommended against his parole last Friday.

As a result of the board's action, he must stay at the Danbury Federal Correctional Institute through the end of his sentence, Nov. 24, 1982, his attorney. Michael Young said.

Novo, 41, tasted freedom last week when papers ordering the release of a "Novo Sampol" arrived at the prison. Officials there released the only Novo Sampol they had - Guillermo - who went home.

A day later, authorities realized the release order was intended to close the case of his brother Ignacio, who was also charged in the Letelier case.

When he was notified of the prison error, Guillermo Novo, short on cash, had to ask friends for a ride back to the prison.

Novo's mother said her son did not let the parole board's decision get him down. His comment to her: "I was free one day for nice visits."