Interesting that this one has both the pre-1912 gauge mark of 156,14 and the typically post 1912 detailed metric marks. 156,14 indicates that the barrel would take a .310" diameter plug gauge but not the next size up, 141,95 or .320". Your .309/.321" measurements mean that it might have been intended as either a "J" or "S" bore but really the primary consideration is whether a fired case will easily accept a .323" bullet. If it will, use those. As long as the chamber is cut so that there's clearance to freely release the bullet on firing the small difference between bullet diameter and groove dimension won't affect anything. The Ch or Cn.15 is unfamiliar to me.