George, I currently have a pair! That is, all of the modules for a pair of 501's. I recently discarded both frames and the built-in TT patch bays. Sorry. I Still have the manuals, power supplies, knowledge and history, updates, etc.
Which version you got? The one with the left and right monitor buttons? Or the one with the left, center & right buttons?
Silver microphone transformer? Or yellow/beige color transformer?
14 pin LM 709 IC chips? Or retrofitted 8 pin DIP with 5534's, sitting in 14 pin sockets?
Coffee proof Auditronics faders with round or square knobs? Or maybe even Penney & Giles conductive plastics?
Jensen output transformer is with the gray cardboard? Or something else?
What else would you like to know?
Ms. Remy Ann David