I haven't looked this up in a while so going from memory but I recall that when only one marking is given it is for bore dia, thus the 18.4 & 18.3 would simply mean a slight variation in bore dia between the bbls & not specify choke. When the amount of choke is specified then the two numbers are given. I have no knowledge of two sets of numbers but suspect you are correct on a re-proof here with enlarged dimensions.


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