Need Radiator Cap for 1936 Cadillac V8

I've been searching on-line for several months for a pressurized (4 lb.) radiator cap for a 1936 Cadillac V8 (2-door convertible). I bought one on eBay but it was not correct - diameter was too small. I would appreciate any leads.

Attached below are the shop manual pages showing the radiator set-up, as well as photos of the cap that came on the car (I suspect it was switched out during the auction) - all the inside components of the cap were missing and the rest of the car was spotless, so I don't think the previous owner sent it to auction that way.