I just disassembled and reassembled the tension assembly on a 15-90, and the finger on the stop washer was in opposite as indicated on the manual, before reassembly, and the dial would only turn from 0 to 6 or 7. After reassembly following the manual the dial turned 0-9, all the way. I think that's the difference.
The thread should move from the tension discs, including being hooked on the little fixed hook, then to the check spring then to the take up arm. Janey mentioned the difference once you get to the needle between the Singer 15 and the White 1514.