Hi there
Henry Nock was a well respected gunmaker in business between - 1760-1810
His arms are of the highest quality and command a premium. For this reason, it was not unusual for gunmakers (particularly Birmingham) to mark their pieces with names of famous makers so that they could sell them for more money. In fact, IMHO seven out of ten guns in the marketplace today sporting such names are period fakes!
This does not mean that the guns are of bad quality, far from it (in most cases). It's just that they were not built by the person named on the piece. I own a number of good quality antique percussion pistols signed by Nock but I can pretty much guarantee that they were not built by him. This does not detract from the enjoyment of owning them. It's just worth knowing if you are going to spend serious money on a gun from a 'well respected maker...' Also, percussion guns appeared on the scene from around 1830. Nock was long gone!
With this in mind, the going rate for a percussion pistol as seen in your pictures would be anything between £90 and £150. This pocket pistol is of standard form and sports slab sided grips (probably walnut) and a turn-off barrel. It probably has two Birmingham proof marks under the barrel.
I hope this helps.
regards
Nick