Here’s a picture of a .38 Long Colt cartridge that came from the estate of C. Kenneth Moore, one of the authors of A Study of the Colt Single Action Army Revolver. Pictured with it is the envelope it was in when I received it, with notes explaining what it is.
The lead bullet has a matching set of marks on both sides, which look very much like someone bit down real hard on it. Otherwise, it appears to be a standard cartridge; the headstamp is F A 4 12. The rim has a flat spot on the edge on each side, one of which can be seen in the enlaged picture of the cartridge, probably resulting from it having been clamped in the piece of equipment that performed the hardness test.