Prior to the 1887 Rules, if a 11 gauge .751" plug gauge will not pass at 9", then it's marked 12B and could be .729" to .750"
Fractions started in 1887 so thereafter if a .740" plug gauge will not pass at 9", then it's marked 12 and could be .729" to .739"
SO if a 12g bore was .739" and the muzzle was .729" there would be .010" of choke constriction.
The Belgian Rules specified muzzle constriction > .008" to be marked CHOKE

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