Before the start of the 2014 season, Penrith added retrospectively 2 numbered players to their list, one of them being John Wilson (531) and the other being Bruce Martin (532).
There was already a John Wilson (82) who was listed as playing in 1972. That John Wilson is listed as playing 1 only match. In 1974, he would be a Amco Cup squad member of the victorious Western Division team (source: The Night The Music Died).
The retrospectively added John Wilson appears to be the one who played at Penrith in 1974, playing previously for and then returning to Parramatta.
I haven't found much about Bruce Martin, save for someone of the same name being awarded a life membership at Cambridge Park Cranebrook (a Panthers junior club) in 1998 as well as being a coach there. May be the same guy. I'll need to follow this up with the club.