The exact age of your shotgun would be hard to determine, however after WWI (around 1918-1920 ) hammers went out of favor in lieu of internal hammers. I would guesstimate it was made in that era.. I don't believe there is any problem with your gun being chambered for 2 &3/4 inch shells. You may want to check the chamber length just to be sure. Remember, a shotguns chamber length is 1/4 inch longer that the shell it is chambered for. The fore arm is either a snap in, held by a spring and a catch or there will be a screw holding it in place. to remove, a firm pull on the forearm will unsnap most common forearms. But I doubt you will find any pertinent information there. The U.S. doesn't use a national proof house, so no proof marks, you will probably find only assembly numbers. Because it is a Box lock I believe it is a Model 225. Others may have more current and accurate information, if so maybe they will post.