PhysDoc:

Before this rifle appeared on GunBroker, I would have told you that this marking was unique to my rifle, a statement that is now obviously incorrect. I would have based this on the fixed rear sight on my rifle, and the absolute need for the owner to know what the regulated load was. Number 437 really doesn't need this caliber designation, since it has a Lyman 48 rear sight. As I speculated, I wonder what 436 has.

I will go to the G&H website and see if they list any other closely numbered rifles in their database, this might be a question to ask Bob Beach.

Is Bob still there, or did he retire?

Anyway, my attempt to answer your question would be, I don't think there are very many out there with the caliber marked this way. Michael told me he had a database with about 1000-1100 (I base this number off my recollections, which for this post have already been proven inaccurate once) pre war G&H rifles in it, it is quite possible he would have known. Does anybody know what happened to his papers, where is Joe when we need him?