2 different but same motors!
There are 2 different motors shown. The first is the round sided HOBART built motor with integral start switch and the slab sided motor is the Emerson built motor that replaced the Hobart motor. This requires a starting relay which should be included with the motor. Make sure it is there before buying the motor.
Either if them will work in the 18,19 or 20 series machines and take the same shaft seal kit. If one of the motors is missing the seal kit, you can still get them from Hobart or possibly places like Marcone or repairclinic.
Hobart currently uses the same style slab sided Emerson motor in the SR24 undercounter machines.Only they use a capacitor to start the motor going instead of the start relay.