From left to right - B50 1956, B51 1959, B55 1974.

B50 motor and on the right the B51 motor.  Differences include the nose mount, and the rotor.

Two B50 motors - early one on the right.

The B50 has the motor on the same side as the arm, the B51 has the motor on the opposite side.

Colours vary but the B50 and B51 are similar until you see under the platter.  Tell-tale is the position of the speed escutcheon.