Jim,
Here is a little pistol which illustrates several points that Fernando has raised. This is a screw-barrel or turn-off barrel pistol opened to show the position of the ball. As Fernando says, this forms a tight seal in the bore by using an oversized lead ball of greater than bore diameter, which eliminates the “windage” necessary to push a ball down the barrel from the muzzle with a ramrod, and makes a relatively high-powered small pistol. This particular pistol is of later manufacture, common design, and decidedly inferior workmanship to Fernando's elegant example. Although marked “London”, it in fact has Birmingham proof marks. It is also marked H Nock, although that famous London maker would not have turned out anything of this rough quality. (In any event, he died before the Birmingham proof house was established). In short, it’s an English pistol that was intentionally mismarked when made to deceive the original buyer with a spurious claim of greater quality than it possessed, or as Fernando says, "increase the image of the product".
Berkley