The 18.3 is the only size mark I see for the bore & choke. As this converts to .720" I highly suspect this is bore diameter & not choke diameter. In this case the word choke means it had at least .2mm (.008") of choke. If actual choke diameter is given on a Belgian then it will normally have two size marks, one for the bore & the smaller for the choke.


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