Ah yes; a 313 gauge calculates to .246" or 6.25mm. I had never before seen a gauge mark or chart which showed gauges smaller than 172.29 or .300"/7.62mm. I had doubts that was a date stamp but didn't think about what it actually was so didn't disagree. If I am remembering correct from 1912 on smokeless proof was compulsory & this one doesn't have them on the shot bbls so makes sense it is pre 1912 at least.

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