Belted Motor and Potted Motor
Potted Motor Machines are highly sought after. They are much stronger than belted motor machines. Belted Motor Machines are not sought after as much.
There are 15 Class and 201's that are belted motor machines.
If you find a Singer Machine with a potted motor than it is either a 15-91 or a 201-2
(Tension dial on the 15-91 is on the end of the machine - Tension dial on the front of the machine is a 201-2 - Either one must have a potted motor to be a 15-91 or a 201-2)