Two FD to M42 adapters, but why different widths?

[Shakespeare]

I recently found these in a case full of breach-mount FD lenses. They appear to be glassless FD to M42 adapters. But why would they be of different widths, with one being three times the size of the other? Wouldn't one of these two mess up the register distance?

S

 

[duhast]

One's a regular adapter, the other (the thick one) is for macro.

 

[Shakespeare]

Thanks for the reply...that just seemed too simply an explanation, but it's probably true.

 

[vintagesnaps]

I have some Canon FD mount that have the breech lock with that extra locking ring; some FD don't have the breech lock so don't have that extra metal ring.

I have some of both but don't have anything macro; I do have a Canon adapter that looks similar to the one pictured on the right so maybe that was used on various breech lock lenses/adapters.