I take it you've contacted locksmiths? If it is a brass prong, you may need someone to machine you a new one. My FIL is passed away, but he was a gunsmith and machinist. He did fine work like that. You may try to think out of the box and find a craftsman/machinist who would be willing to work on it.

We are fortunate to have smithys here, and I've had antique bronze lamp fixtures/old locks repaired in those places.

You might find help - either directly or referral - through a local architectural salvage company.

There is a fabulous salvage store in Richmond, Virginia called Caravati's that does hardware restoration.

Here is a link that might be useful: Caravati's Inc Architectural Salvage