Singer site implies all JB are 1936
http://www.singerco.com/support/mach.../double-letter
It's a model 15-something. Only the 15 series has the thread tension out on the end like that. Might be called a 15k-something, since it was made in Canada not USA. 88 is treadle, 91 is potted motor, maybe 15-90 is belt-driven motor? If the motor is aftermarket and not Singer, then probably is was a 15-88 treadle head that they motorized.