IMO, the pistol is a pocket or muff, bar hammer pistol, possibly made by Allen & Thurber in the 1840's. If it was made by Allen or Allen & Thurber the name of the company on the barrel.
It's also possible that the pistol was made by any one of dozens of other companies during the 1830-1860 time frame. Almost all of these pistols will have the makers name stamped on them.
In 2007, Flayderman's Guide to Antique American Firearms said a Allen & Thurber pistol like this in good condition was worth $200-$500.