What is this one?

[kxxr]

Seems this one has the GJ crowd stumped. They are pretending not to be interested. Or maybe they are not interested. Only one guess there, 2oolhound thought it might be for putting wire whipping on something (I don't know what that means) or for fly tying. I can see how it would assist in looping wire or string around an object, but ...
Anyone here know? There are no markings on the tool that I can see.

[fliffy42]

maybe a saw handle for two-man use?

[kxxr]

I could see that. The grooves would help keep the handle from wanting to turn ....
So, I guess I need to keep my eyes peeled for another one for the guy on the other end.

[RWalters]

I'm going with fliffy42 on this one. I believe this is a helper handle for a one man crosscut saw. Not the big two man saw like the lumberjack cartoons, but one that looks like a much heftier version of a carpenter's saw with a wooden grip (though usually without a top horn). The handle could be attached near the grip to allow two handed use, or at the end of the blade to allow another person to help make the cut. I couldn't find one that looked exactly like yours, but the general concept is the same. The one man saws had holes in the blade at each position, and the pin in your handle would go through these. The hook shaped part retains the pin. That's my guess and I'm sticking to it.