Around 1953-55 Singer gave a few of their older models an updated look. It was more angular, and I think it was considered more stream line. Some of them were aluminium cast like the 201K23, some of the 15s might be too. The 99 was given a new look and model number 185, but remained the same machine internally. If your 15 has the new look, and has the belt driven external motor it's a 15-75.
With model 15s there are a lot of variants and differences, it was made for a very long time and went through a series of alterations and improvements along the way. It can be a bit of a fuzz to detect all the details; bobbin winder, bobbin case, top tensioner, with or with out a stitch lenght lever, the early version have extra large heavy hand wheels,... In addition the same machine came as treadle, electrical, hand crank, and later on both with belt driven and gear driven motor. It mostly when you search for replacement parts you need to fuzz about the sub-numbers.
Last edited by Mickey2; 04-20-2019 at 07:17 AM.