Want to spend a few books for a reference? http://www.cornellpubs.com/old-guns/item_desc.php?item_id=1304
Going back a ways, there is a lot of gun history behind your revolver. Ethan Allen (no relation to Ethan Allen and the Green Mountain Boys) had been in the gun business, under his name, Allen & Thurber, and Allen & Wheelock (think pre-revolver pepperbox pistol days) Ehan had no sons- two daughters. When he died in 1871, his sons-in-law, Sullivan Forehand and H.C. Wadsworth ran the company under their names until 1890 when Wadsworth died, name changed to Forehand. Forehand died in 1898, and Hopkins & Allen bought the firm, ran it under the Forehand name until they went belly up in 1902.
Odds are your gun was made 1890 and 1902, likely shortly after the name change. STRONG likelihood it is meant for BLACK POWDER .38s.