a 1925 or later example. Lots of questions , but luckily we have lots of examples.
Can we now say that 12 over C in the diamond, prior to 1925 means a 12 ga chamber that could be anything from 2 1/2" to at least 3 1/4" [probably more] . Then after 1925 the 12 over C in the diamond would be proper for chambers up to, but including , chambers of 3". Then the 12 over LC would be used for longer than 3" and would also be marked with numerical length ?
A later thought is that the gauge over C in the diamond just meant a gun with chambers [any length], Earlier there were centerfires and of course muzzle loaders that did not have chambers, thus the C designation under the gauge .