When one sees only one diameter mark on a Belgian proofed gun as this 18.2 mm mark, then I believe that is the bore diameter & not a choke mark. Depending upon the actual date of proof choke bore guns will be found marked Not for Ball, Choke or with bore diameter over choke diameter as in 18.5 / 18.2. In this case, then 18.5 would be the bore diameter & 18.2 the choke diameter.

As this gun carries none of these marks it may well have been cylinder bored in both barrels.

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