Difference between 1955 and 1957 Starters?

[gnarlycreature]

I have a 1954 Chevy 3600. Original 235. Normally these come with a 139 Tooth Flywheel.

The previous owner did a partial conversion and has put a 168 tooth Flywheel in the truck. It is still a 6v vehicle. He modified a 6v starter with the correct bendix and armature. I am having problems turning this truck over with the starter.

I'm going to complete the 12v swap, but I'm having issues locating a complete 55' starter. I do not want to convert this starter because I am concerned there may be play in the bendix and armature. I believe this play may be contributing to the issue.

57' starters are available. Is there a difference between the 55' and 57' starter?

Thanks Guys!