Interestingly, the #31 & 31RH looked exactly like the brackets on my 54 Chevy sedan back in the 70's, so I checked and they are the same part number. On the sedan they went through the doors however, which is not a great arrangement. Slamming the doors has a tendency to jar the spotlight head, which damages the parking nub in the handle over time.
Knowing what else this part was used on might prove helpful in finding additional number 31 brackets. This would have been used from 1949-1954, and probably on Pontiacs as well. Here's how they looked on the sedan:
