NOTE: the prices below come from current listings and auctions as of Feb 2013.
I think you have a Stevens Diamond #43, the second revision (the first had a brass frame.) .22 freehand target pistol made from 1896 to 1916.
The photo lacks sufficient details to be absolutely certain of the condition, and no shot of the other side or top. I can't really tell if the barrel retains any of its bluing, or if there is any pitting.
Never-the-less, it is certainly "Good" and might well be "Fine" depending on the condition of the barrel and how much blueing is retained.