A chamber gauge does come in pretty handy. Not very expensive.
But in the case of that Greener . . . being marked "not for ball", that would make it a 19th century gun. Too old to have the chamber length stamped as one of the official proofmarks. (That didn't happen until 1925.) But on a gun that old, the gun would almost certainly have left the factory with 2 1/2" chambers . . . although there's always a possibility, on older guns you find here in the States, that they have been lengthened to 2 3/4". 2" wouldn't be possible. The 2" 12 came along later, well into the 20th century.