The 12 C in a diamond shows that it was Proofed (for Black Powder) for 12 bore cartridges rather than as a 14 bore gun.

The 13/1 and 14/1 markings are the bore sizes at 9 Inches from the breech.

It appears to be in exceptional and untouched condition for it’s age, but I would still seek the opinion of any experienced gunsmith before using it.