You are right GBeer that this part comes from a Gowland camera. Unfortunately Peter Gowland passed, so getting parts is not quite so easy anymore- if indeed there are any parts left. My part isn't cracked- it's just a bit deformed inward in spots making focusing difficult.

Thanks for the responses everyone- I will probably end up getting a machinist to do this if I can't source a part. It is encouraging to know I could do it myself if I just had a tablesaw though..