I found this on ebay, and wondered if it's really a clock key?
Thanks,

long handle

Perhaps a home-made mainspring let-down key?

I would agree with Dave on this one, looks like a let-down key to me.

Would that be to unwind a clock?
I figured it was a home made model.
Thanks Guys,

Yes, to unwind the mainspring before disassembling the movement. To do so, you put a key like this on the winding arbor, hold it tightly with one hand and release the click spring. Then you release your hold on the key slightly so it starts to slip in your hand and the spring unwinds in a controlled manner. The cylindrical shape of the handle allows this controlled slippage -- something you couldn't do with a regular winding key.

Dave,
Wow, I was just reading how to do that very thing!!!!!

Than you so much.