This is the closest I've been able to come...1,537,631.
This total number though will probably never be correct. There are untold numbers of "D" guns out there that we don't know of yet as well as "duplicate" guns that never got a "D" stamp (you're right....how would one know they are a duplicate!).
Then there are all the guns that were scrapped, or even skipped serial number sequences, small and large that had no guns made at all.
Chet15