Yes, it would be interesting to see a photo of an original SM420 backup switch?
Novak Transmissions website:
"The last year of the SM420 (1967 [maybe 66 & 67])
had a backup light switch installed in the top cover to meet that year's federal mandates." Novak installs a switch on their transmission rebuilds but it does not look like they sell a kit for others to install. Maybe they install a conversion as described below.
Old Stovebolt.com thread on adding a back-up lights switch. Here's another conversion. 
