I kind of expected someone would know exactly what that was, and depending on who that person was, would ridicule me for not knowing something so obvious.
I have continued to google and still found no close match, but I think it has to be intended to shape metal. It is obviously a one hand tool, but with a very heavy head that narrows to a small face so the strike has maximum force on a small area. So, moving metal around to reshape it directly, rather than hitting a punch or chisel indirectly, which would be safer and easier with a larger face. Some kind of forging blacksmith tool - although I found some blacksmith forums without an example.
The face angles are between 10 and 20 degrees off the axis of the handle, but they are in opposite directions, so the choice seems to be for the effect on the object rather than the arc of the swing.
This guy's workshop was a total disaster, no way to tell what he worked on, but most of the equipment that was left seemed to be woodworking tools and materials (paint, lumber, dowels, etc.) It wouldn't surprise me if he found this hammer in a yard sale a few decades back and thought, wonder what that is, gotta grab it.