The breech block is stamped U.S./Model/1873. The 3 is stamped deeply enough to make it look a little like an 8, and the bottom of the L is missing, making it look like an “I.” Because of that and its flatter arch and rounded shoulders below the hinge I think it’s the sixth type of breech block. The square end of the shaft is visible at the end of the cam latch, so I think it’s the third type. The shape of the thumb piece makes me think it's the second type.
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