Thanks for the pictures, those must be 50mm fenders? I hate to tell you that the builder forgot to put a binder bolt for the seatpost

I measured the width at the bolts, it's 1.9 inches or a little over 48mm. This dimension is probably the same on everything other than RAIDs. I'm going to use 40mm Berthoud fenders. Just a little narrow, but they seem to work and they are bomb-proof. I think you could get a 50mm fender higher up with just the tiniest bit of blacksmithing. My main concern is that the fender not keep the brake from opening far enough to clear the tire. I would like 15mm of clearance in combination with keeping the brake shoes fairly close to the pivots for mechanical advantage. Hope to be able to mock it up on the frame today if I quit mucking about on the internet and go work on it.