I don't think these are super rare - I've seem some that were made in 1931 and also 1932. Based on the ismacs information, this may simply be what Singer did for the very first potted motor 15's they produced. When you look at the production runs, prior to 9/31 they are large. Then about every three months from Sept 31 to early 33 the runs are all 5,000. Counting up, if these were all "silver" there would be about 40,000 of them made. Based on how many I've seen, i think that would be more reasonable than only 5,000.
In early 1933 the runs go back up to larger numbers, and this is when Singer started making the same model with the potted motor in the non-potted motor version.
So - those out there with Singer 15-91's, anyone have a non-silver one with a serial number earlier than AD 502,700?