Has been a while since I looked up Belgian proof marks, but as I recall the 18.3 & 18.2 are bore diameters. Most likely taken before & after final polishing. Thus at final proof, it would have had the 18.3 (.720") bores. The word choke as I recall means that barrel has at least, but not limited to, 0.2 mm (.008") of choke. The choke should now read 18.1 mm (.712") or smaller

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