Don, I think Jim is talking about a different tool, also often referred to as a Mortise Ax.

Jim, I looked into this pretty deeply a few years back when I acquired a few of them (Yes, I have an Ax problem, now fully under control) The name is a bit of a misnomer, I don't believe they were intended for use in mortising, but are a tool used by Logcrafters from the Nordic countries, used primarily to cut a dovetailed slot in the endgrain of window & door wells to let in what we know as a buck.

Gränsfors Bruks still sells them, and lists them under log building tools and I know they are still used in this way by traditional logcrafters from that area.

I found both of mine in areas settled by folks from those regions where there was a long logbuilding tradition.