Otto, the original tail lights use single filament bulbs, and 6 and 12V sockets are the same for those, with the bayonet pins at the same depth on both sides.
Dual filament bulbs use the same diameter socket, but with the bayonet pins offset so that the bulb only goes in one direction, aligning the contacts with the proper wire. Again, 6 and 12V bulbs are interchangeable (as far as the socket is concerned.)
Dual filament bulbs are 1154 (6 volt) or 1157 (12 volt), and single filament bulbs vary with candle power. The 48-51 Shop manual calls for 3 CP #63 tail light bulbs and 21 CP ((#1133 is advertised as 32 CP) brake light bulbs. I don't know the 12V equivalents of either the tail or stop light single filament bulbs.
