Thanks for your input guys, but I think my pics misled you. This tool is huge.
I think it is too big to be a stone mason's tool. I should have given some reference in the picture for size.
In my wrenching days my forearms were bigger than most men's necks (>17") biceps were more than 20". I could curl a set with 135 lbs., regularly with 120 lbs.
I couldn't have swung an 8 lb hammer for very long.
Here is a picture with other hammers:
Included are two goldsmith hammers, a Peddinghaus and a Fretz, a standard masons hammer, and my largest Fretz metal pounder - a jewelers sledge (for silver, gold, copper, and brass.)
The big Fretz SH-1 has a 1ΒΌ lb head. The largest stone masons hammers I could find in old Stanley catalogs were 4 lbs.
This tool is 8 lbs. At it's widest it is 1-5/8" wide, 2-9/16" tall and 8-1/4" long. the cutting edge is sharp, but widens out too fast to be an axe. The striking face is 2-3/8" x 1-3/8" This tool is stout.
I am pretty sure it isn't a stone masons hammer, I have a dozen or so examples of those.
Not really a tool to be swung, if so I would think it would have been made to accept a larger handle, like a sledge hammer. A big sledge is 10 lbs. At 8 lbs it is of sledge weight.
I am still of the hypotheses that it is a hot cut tool, made to be held by one person on hot metal and struck by another smith. More a handled chisel than a hammer, really. I am basing my guess on you tube videos of smiths making damascus steel. Many use a tool like this on top of the steel, or a hardy on the bottom side, to cut a hot iron billet before folding it back to re-weld.
Another guess is some kind of wood splitter or maul, but it is bigger than most of those I have seen commercially.
Chilly