Le Mans filler cap: what is it?

Of the many reactions I received after having secured the Le Mans Mini Marcos, Tom Wiggers was the only who said he could help me with the car's huge fuel filler cap. Tom told me it was sourced from a Ferrari 250 SWB and that he had an original one himself plus had some copies made. However, I had a close look to compare the two and am convinced it's not that. Tom's reaction: "You are right indeed!"

The original thing has to be approximately 150 millimeters in diameter, making it anything but your average filler cap size wise. It also looks as if there is a brass or bronze lock on it. I have attached all the reasonably good detail shots that I have from it, and also attach the photos of Tom's repro-Ferrari-cap to show it's not that.

Question now is: what is it?

Ferrari 250 SWB filler cap - definitely not the one used on the Le Mans Mini Marcos

The size must be about the same though, but all the detailing is different