I wonder if Singer sourced their modern 15s from the same plant making the Sewmaster machines. That would help explain how this issue got as confused as it is. Half or more of the parts on your Sewmaster may have made with the exact same machines and tooling as the parts found on a machine that was actually authorized by Singer.
To me your Sewmaster is like a 15 clone with the top tension moved to the front. The bottom half of the machine and the stitch regulator are certainly based on a model 15.
Rodney