The original Belgian proof was 1910 - 1921 (there is no lettre annale) with a bore of 18.2 = .717". 18.3 = .720".
The Belgians used a plug gauge like the Brits so .719" would be marked 18.2 ie 18.2 gauge passes but 18.3 does not.
13/1 would not be a Belgian mark and is .719"
The 12 in the circle is German for the 12g chamber dimension
'JJR' looks to be German and I don't see a Belgian maker's mark.

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