What do you think this is for ? Its about 3 3/4" long.

The base looks like it fits in a drill brace. The hammer head is similar to those in knife tool kits.   

I agree with Yaddas description and thoughts.
The top end does looks like a small tack hammer but as to why one would want to put it into a bit brace and then try to hammer in a tack ? Makes no sense to me.Maybe somebody can jump in and help us out with this. If I saw one like that I'd grab it for sure, it has a very high cool factor.

Maybe a handle was molded around the key at one time?

Early attempt at a power hammer?

And there is no notch for it to clip securely into a handle like some old multi-tip screwdrivers.
I bet someone will come up with a good idea, though.

Maybe a "3 way" tool ? Tack hammer, tack puller and a "tee" handle to engage some sort of thingie... like perhaps a web stretching winder for upholstery work ???

Joe B

I think the key is for a locker of some sort, not a multi tool as someone has suggested, it wouldn't stay in the handle with this taper.