The 1 oz proof usually indicates chambers of 2 3/4". The 12/C does not indicate an exact length of the chamber. 12/LC mark indicates a chamber length of "Over" 3". That said, guns with 3" chambers exist both with & without the LC mark. I believe these marks would indicate manufacture between 1887 & 1904


Miller/TN
I Didn't Say Everything I Said, Yogi Berra