Your pair of pocket or travelling pistols were made by Theo. Richards in London around 1775. There do not appear to be any hallmarks on the silver butt-caps that would enable more accurate dating, although the caps are almost certainly the work of Charles Frith. The proofmarks are what are known as ‘private proofs’, quite common at this time and often mistaken for later Birmingham marks. You are correct in assuming that they have been converted to percussion from flintlock, but actually they are ‘boxlock turn-off’ pistols as opposed to ‘Queen Anne’ types. True ‘Queen Anne’ pistols have side hammers and exposed frizzen springs.

Unfortunately, even though it is part of history and was very common practise, the conversion to percussion does effect the value. In an English auction as they stand I would expect an estimate of £500-800, with a realised price of around £650-700. In fairness to you, this sort of price should be readily achievable in the U.S. as well.