Its a powder tester. The cogwheel is part of the muzzle cover. And there is a spring that bears against the cogwheel. When its fired, (full to the muzzle), it blows the muzzle cap as far as it will, where the number on the cogwheel is registered. Then there is the lever under the trigger guard that takes the spring tension off so you can reset it. There is also a flat bottomed drill bit in the other end of the nipple wrench (covered by a cap) that lets you make sure all the crusty stuff from the last shot is gone. Doesn't hold much. I/8" bore and 1/2" of bbl. But it sure is a DO-DAD!
A charge of FFFG went to number 3.