Callen,
The charts that tell me the model number by serial number do not go to the dual letter numbers.
All I can find out from the charts I have is that JB is 1936 and made in Saint-Jean-sur-Richelieu, Québec, Canada.
If the machine has a potted motor like Carolines but has a tensioner on the front it's most likely a 201-2.
If you post a pic we can ID it.
Joe