Hinckley's mail sifted, prison official affirms

The head of the psychiatric unit where John W. Hinckley Jr. was housed acknowledged that Hinckley's mail was routinely ''shaken down'' but denied that his private writings were read by prison authorities.

The testimony from the unit manager of the mental health unit at the federal prison in Butner, N.C., where Hinckley underwent evaluation came as defense attorneys went to court seeking to bar use of seized writings at Hinckley's coming trial for the attempted assassination of President Reagan. His lawyers also want to exclude from the trial statements Hinckley made to psychiatrists.

Defense lawyers said a confiscated private memo - which was forwarded to the FBI - contained Hinckley's ''most private, if not secret, thoughts about his legal situation'' and that it should not be used in the prosecution.