US Civil War horse bit. It may be Model 1863 Artillery/Cavalry bit (looks similar to others I have found on-line)? I believe the bit is made of iron since it is pretty rusty. I have read that the US emblems on each side are made of brass but I can't tell for sure. It does have the US stamp on the inside. I can't see any numbers anywhere, but the rust may have covered them.

6 inches wide, 9 inches long, 2.25 inches high