Hiya Matt.. I don't know who may have originally supplied A/S with the bumpers, maybe they were made in house....
But the spare tire carriers are a simple setup consisting of two steel plates, one on the face of the bumper, one on the backside, that are held together by four long carriage bolts/lock nuts/nuts... sorta squeezing the bumper. On the outer steel plate is welded a piece of square tube, about 3 feet tall, and to this there is bolted another plate that the spare wheel itself bolts to.
I bet you can get a "universal bumper mount spare tire carrier" at just about any RV supply house, Farm and Fleet, Tractor Supply, etc...
There's a company.. "Briz Bumpers" they may be able to guide you in the right direction..I mean, they may know where the original style bumper material is available.
Briz makes some cool ribbed bumpers, though... I might throw some on my Argosy 28!
Slick and Kathy