Many thanx Harley! The history and linage of the Mk IV is pretty easy to find from both the web and published books. But as of yet I can't find the production data that would tell me the year of manufacture of this particular gun. It has an all numerical sn, so it is pre-end-of-WWII, and it is stamped "WAR FINISH", so I suspect it was made in the early 40's, but I would like to track it down closer than that.
I would also like to locate a source of parts (NGP has none), as the cyl latch spring is broken. If I could get, at least, a detailed description of the spring, that would be great, as then I would make a replacement.