[rhmc24]

Here's the skinny - as someone touched on above. It is to allow the pan to be primed while the frizzen is open and the hammer down. Most flint locks when primed and 'ready' must have the hammer at half cock to carry it safely. For long storage some people think that constant strain on springs is undesirable. So - this device allows the gun to be primed for quick use, with the hammer down, frizzen open and springs relaxed. It is a rare feature, second one I know of.

French, I agree.