Hope I'm not breaking a rule posting a link to an active one... but what in the world would you use this for??
http://www.ebay.com/itm/Unusual-Old-Round-Head-Hammer-Blacksmith-Body-No-Name-Used-/160851723183?pt=LH_DefaultDomain_0&hash=item25738283af
Obviously the anvil for a "nut cracker" ;-)
I have seen something like this before that is held on something and struck with a regular hammer, sort of a protector or a larger striking surface. When they used to set type for printing, they used something like this to level type without hitting it directly. Just a wild guess.
Is that held behind auto panels when working it back to shape with another hammer? Before the time of mix-up bog of course.
Vaguely like a bookbinder's beater or hammer - see: http://jeffpeachey.wordpress.com/tag/bookbinding-hammer/
Not bookbinders, I don't think. Notice all the bookbinders hammers have very rounded edges around the face.
Maybe it's time to contact the Hammer Museum:
http://www.hammermuseum.org/
They have a contact button there, and have identified a couple of difficult hammers for me in the past.
No, not a bookbinder's hammer, but I still have a vague feeling it's for bashing leather - saddlers or harness maker???
Update - probably a red herring - it's a saddler's mallet (in wood) that I am thinking of - do not know if they made them in steel....