Actually, there was a backup switch available for the SM420 in 1955 on the 2nd Series trucks. It consisted of a collar with a tab for the switch that mounts to the shift tower, then a cast aluminum peice the mounts to the shifter to activate the switch. It is all external of the transmission. The same swithc was also used with the 3 speed manuals, as well as the Hydramatics. If I can remember where i put the instruction sheet that came with the NOS backup lamp kit I used on mine, I'll scan it and post.
