The possible difference is that the very first group of the 15's with full reverse came out with a silver tone cover for the stitch length circle. That would have been 1932 and into 1933. I don't think it was a special run, just something Singer started with too highlight the change, and then decided to go with something simpler.
1933 model 15-91
this is a 1952 model 15-91
If the 15 is before 1932, it probably would not have reverse. There is a knob on the pillar is to adjust the stitch length, but it only goes forward.
I'll add - Singer still made some of the older style when they first came out with the newer style. this is a 15-86 from 1935. Very much like the earlier 15-30, except the stitch length lever works opposite.
