I have a P Webley & Sons hammer gun in 12 ga., single bite underlever, Stanton 367 locks, and a keyed forend. This, that, and the other, I have always thought it to be ca. 1873. I don't know what PW&S's trajectory was in the late 1800s, but it appears they were turning out a pretty good volume before merging.

Lagapus - do you have any more of a refined idea of when #6541 might have been made?