Gerry, I'm doubting Francotte too, for several reasons. The AF we're talking about would be on the barrel flats along with the other proofmarks, not on the receiver. Under a crown. That's the Francotte maker's mark. And the SN simply does not fit into the Francotte range for either a medium or best grade gun. Best grades never got out of the 20,000 range, and medium grade would have been 1920-21, which does not fit with the proofmarks.
Nevertheless, it is a nice gun. I do wonder where Christie's got the "2IN" thing, because there's no such thing as a 2" 16ga.