It depends upon what kind of local plumbing shops are around and how they stock supplies.
Easiest is to replace the entire assembly, though many plumbers and some older, independent plumbing supply shops have various misc parts.
The many clawfoot tubs used a 1-3/8" threaded style.
buy new and screw it in with putty on top gasket underneath. fine threaded would be 1 3/8 (1 5/8" od at threads) 1 1/2 will be 1 7/8 inch on the od . eventualy some of plumbing makes sence.
how do you know what size to order for a clawfoot drain? My piece on the bottom of the tub is 6 inches long. That's the piece the strainer screws into.
I told you what size as did Breplum? looks to me like its a broken spud and we both told you what to do ? you got 2 pieces in the photo one is busted. re read my post with sizes measure the od on the bad part if its fine thread it almost surely is 1 3/8inch which is odd because it actually measures about an 1 5/8" . this is all i can tell you