I'd say you are right on the Bentley and Playfair origin. The Deeley use number could tell a bit of a story if Westley Richards files were available. They required delivery of such guns to their premisis for inspection and stamping, before the guns were finished up. Lots of Birmingham guns with London proofs for the reasons one might guess. It could have just been handier, or requested.

http://www.bakercollectors.com/