The September/October 2016 Muzzleloader Magazine carries an article by Mike Nesbitt about the GRRW CA. Here is the first fullstock Hawken built by Carl Walker. I think it is a .62 with a 36" barrel and interchangeable locks.
The screwdriver slot in the flash hole liner allows its removal to be replaced with a drum and nipple for the caplock. This is that famous GRRW stock finish, linseed oil.
