At the point this rifle was made, British proof law only applied to the "greater London" area... outside London (and parts of Middlesex county), there was no requirement for proof. Many B'ham guns have private proofs, but this was because the customers required them rather than that they were required by law.
Even after the founding of the B'ham proof house, proof law applied only to England and Wales, not Scotland or Ireland although, again, this was more a technical point. Its very unlikely many unproved guns were sold in Britain. In fact, there was no market at all for the really cheap guns that were exported in such huge quantities so there was no incentive to make them for domestic sale.
As far as the wrist is concerned... I had a Jover officer's fuzee made ca. 1770-1775 that had nearly the identical device. However, I attach no importance to that... these bits were made by others and bought in by the gunmakers. I know of at least two other nearly identical Jover's - one was in the Warren Moore collection and another belongs to Don Troiani. I also know from a primary source (the record of a trial at the Old Bailey) that Jover bought these pieces and did not make them.
My own feeling is that Mike Brooks hit the nail on the head with this one. Gamekeepers, (who were senior and very responsible employees of large estates and even had quasi-legal authority to arrest and prosecute poachers), often had very nice, albeit plain guns. Aside from some deer park shooting there was almost no use for a rifle in England in the 18th century. It was also reasonably common for a landowner to pass his older guns on to a gamekeeper... when you see an obviously superior gun that shows evidence of a great deal of use, this is the most probable reason.