Be very careful about getting that limit screw out of the clamp body. I would suggest taking a picture of it and starting another thread to get many suggestions on how to remove it.

That said, worst case scenario, it probably is one of the cheaper parts to replace. You can probably just buy a used clamp band and attach your original flat bladed derailleur body to it.

Definitely check that derailleur patent stamping, it may be worth more than you paid for the whole bike