18.6mm = .732", 18.2mm = .7165". Nominal 12ga bore dia of .729" = 18.5mm. Unless it vastly over size bores those sound more like Cyl & choke to me. Then again they may just be the bore dias & one bbl has an undersize bore. I haven't looked up the Belgian marks in a while but seem to recall that if the bbl as only one mark it is bore size. If choke is marked there should be two marks, as for instance 18.5, 18.2 both on the same bbl, then it would be choked by .3mm/.012".

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