The 6 volt starter main housing or field frame diameter is the same from one end to the other.
6 volt starter also has a clamp on band on one end to cover the hole were the brushes are located. Illustration A

The 12 volt starter main housing or field frame diameter dips down a little on one end and it doesn't have the clamp on band . illustration B

Check attached

xfiles

Attachments
starter 6 and 12.jpg (44.1 KB, 124 downloads)
6V 12V starter.jpg (32.41 KB, 110 downloads)

1954 Chevrolet 3105 Deluxe Panel Truck
w/Factory HYDRA-MATIC