Last week, two handwritten letters arrived from Garrett B. Trapnell, an inmate at the federal penitentiary at Marion, Ill., stating that an item in this column two weeks ago "had caused . . . serious danger to my life."

He's talking about the piece describing how federal prisoners had lost what was in effect their free mail privilege and now are limited to five free postage stamps a month.

The Federal Register of June 30 (page 44220) had carried the final rule on the matter, although a temporary rule had been in effect since June 1979.

In citing illustrations of prisoner abuse of the privilege that led to the change, the Bureau of Prisons noted in the June 30 Register that "one inmate sent over 150 cards and letters to people he did not know asking for money." A government official, asked about that reference, said it described the activities of a federal prisoner who was running the president. He added that it may have been "the last straw" in convincing prison personnel that the free stamps had to go.

Trapnell is serving a life sentence for a 1972 plane hijacking. He achieved notoriety in another sense in May 1978, when a woman was killed after commandeering a helicopter at gunpoint and attempting to free him from the Marion penitentiary. In December the same year, the woman's daughter hijacked a plane over Kansas City and demanded Trapnell's freedom. She was captured by FBI agents.

Now Trapnell is running for president from his prison cell. He denies ever having solicited campaign funds using the prisoner mail privilege, and sent aong documents that indicate he tried unsuccessfully to get prison approval to buy a postage machine in March 1979 to handle his campaign.

The Federal Register information, when linked to him Trapnell wrote, is designed to "aliente me from 30,000 federal prisoners" and thus "put pressure on an inmate who only wants to speak out against real tyranny in prisons."

The Washington Post item, he wrote, "reinforces any half-crazed lifer into believing I'm responsible for the cancellation" of the free mailing privilege. b

Bureau of Prisons officials have told Trapnell the elimination of the privilege was under consideration for months before his presidential campaign began -- but will say no more because the matter is in litigation.