As Marty posted, GM did not make those conversion panels, but a dealer could have ordered the conversion panel from another company. So, the owner's father could have easily bought the vehicle from the dealer. The dealer had a Silver Book (Special Equipment for Chevrolet Trucks) which they showed customers who were interested in a specially-equipped truck (a non-stock truck). The kind of conversion you are describing has been described by others for use as ambulances, logging-crew trucks, military-crew trucks, and school buses.
If you are interested in seeing illustrations from a Silver Book, I'll scan and post a few pages from an early-50s edition.
