You have made a very nice discovery! I never knew before that "Charlie Chan in London" made use of exit music. At least one other Charlie Chan movie, "The Black Camel," had exit music at the very end. What you have told us makes me think that others from at least the early series had similar music that was heard against a blacked-out screen.
If you are ever so inclined as to make VHS or DVD copies from your print, I would be very happy to be one of those in line to buy one from you!