Yes, it's a special hammer, for people without 3 hands ;-) In upholstering the craftsman needed to hold the hammer, the fabric *and* a nail. So a hammer was developed that would hold the nail in the slot). The first hit would 'tack' the fabric and the nail, then the tool was rotated 180 degr, using the hammer side to drive the nail into the wood. In Dutch, the specific nail is still called 'tackje'. Modern versions of this tool are magnetic at one end. I'm sure this tool is antique, last time I saw it being used was by my granddad, repairing a chair around 1955.

I'm pretty certain it's a tuning hammer for tuning pianos and maybe harps-any instrument with a string sound board-you need a lot of torque to tighten the larger strings hence the 90º. Your pics are fuzzy but the end should be square-the tuning pegs on most pianos are square-not hexagon in shape although some have star shaped pegs. It appears to be one of the older ones based on its design and wear. The slot is for checking the tension of the string.

References:

http://www.amazon.com/s/?ie=UTF8&keywords=piano+tuning+tool+kit&tag=mh0b-20&i...

Reason:
Thank you for your opinion, but this tool does not have a keyhole inside the tip like piano hammers. I posted better pictures if this will help you ID it.

I'm pretty certain it's a tuning hammer for tuning pianos and maybe harps-any instrument with a string sound board-you need a lot of torque to tighten the larger strings hence the 90º. Your pics are fuzzy but the end should be square-the tuning pegs on most pianos are square-not hexagon in shape although some have star shaped pegs. It appears to be one of the older ones based on its design and wear. The slot is for checking the tension of the string.

References:

http://www.amazon.com/s/?ie=UTF8&keywords=piano+tuning+tool+kit&tag=mh0b-20&i...

Reason:
Thank you for your opinion, but I was unable to find one that looked like ours. I've added more and hopefully better pictures. I hope this helps.

Yes, it's a special hammer, for people without 3 hands ;-) In upholstering the craftsman needed to hold the hammer, the fabric *and* a nail. So a hammer was developed that would hold the nail in the slot). The first hit would 'tack' the fabric and the nail, then the tool was rotated 180 degr, using the hammer side to drive the nail into the wood. In Dutch, the specific nail is still called 'tackje'. Modern versions of this tool are magnetic at one end. I'm sure this tool is antique, last time I saw it being used was by my granddad, repairing a chair around 1955.

References:

http://greenspotantiques.com/upholstering-tack-hammer-galt-ontario-metal-and-...