The man asking Perry Mason for an autograph mentions autographs he already has. The names he mentions are of actual killers, and some serial killers. The ones listed are: (William) Edward Hickman, Gordon (Stewart) Northcott, Louise Peete, The Lady Strangler and The Attic Man (Theodore Edward Coneys).
The Bonus Army was a group of 43,000 demonstrators who gathered in Washington, D.C. in mid-1932 to demand early cash redemption of their service certificates.
All courtroom scenes were set in Department 42 of the Los Angeles Superior Court. In fact, there is one Department 42 in Los Angeles County, and it is in Central, in the Criminal Courts Building in downtown L.A. All of said courtroom scenes were filmed at Paramount's Stage 27, and the extras' holding area was in Stage 28.