Yeah, guys, I need to get myself photo-compiant. No doubt.

Joe - You and I probably have the same thought line regarding the key/wedge. To my mind, most of the makers were dabbling with something more sophisticated by about 1875, thus my early-1870s surmise.

400 - from the list of proof marks in the Blue Book, the gun dates from 1868, but it is indicated that that mark was used into the twentieth century (no help). Is there a more comprehensive list of marks available?

Lagopus - My understanding is that the Purdy Thumbhole was a double bite and this is just a single. From what I have read, the single bites were among the gunmakers' first venture into the locking of a breaktop gun, and the genre is a wide ranging hodge-podge of everything from good to trivial. The Webley's is a straight forward design, and fairly elegant - just a bite away from Purdy's double. Crudington & Baker don't describe or illustrate anything close to it.