12/LC in a diamond is a bit ambiguous. Some sources say it is for chambers of 3 or longer, while others say it for chambers longer the 3".
I believe that uns having 3" chambers are known to exist with either the 12/C or 12/LC mark. Either way, the 2 3/4" chamber is marked with the regular 12/C mark.


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