Interesting! Can you provide a source for that info?

Thanks!

UPDATE: According to this European proof marks source, the marks on my gun: crown over "BNP," indicate a post-1954 Birmingham proof. So it looks like LWCmder45 is right. Interesting that a close look with a magnifying glass doesn't show bright marks on the sides of the stamping, which may indicate a refinish after the proof marks were stamped in.