.... I would think a cracked receiver would be noticed...
I gathered it was reblued with the coppery looking finish, but as far as a crack in the recv'r is concerned,,don't depend on the last guy to cull a bad one out for you. Some don't know,, some don't care.
The crack(s) aren't gapping openings, at least in the few I've seen. Hair-line would be a common term and barly that. Check carefully and w/magnification is my advise. It's just one of those things on the check list when looking at a sporterized 1917.
The mentioned relief cut on the old bbl right up close to the rec'vr is the easy way to avoid any of the problem w/removal. But the gun in question is way past that point.