The muzzle dimension over bore (but sometimes the opposite) was used 1910 - 1924 (but some earlier guns were so marked)
Starting in 1922 there would be a lettre annale on the barrel flats, and after 1924 the barrel was usually marked CHOKE (indicating muzzle constriction > .008") and the bore only

Did you switch the numbers, and if not the greater choke was R
18.4 mm = .724" and 17.9 = .705" so .019" choke = modified
18.5 mm = .728" and 17.6 = .693" so .035" choke = full

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