Every mark you have described is related to the proofing of the gun. Both the gauge mark and the barrel weight are of the style that was used before 1924.
The barrels weighed 2.738 lbs at the time of proof.
20.6mm is the bore .811".
Choke 17.5mm = .689", 18.3mm .713""
The service pressure during this period for a 12ga was expressed in a load. In this case as 5.8 grams of powder and 35.5 grams of shot. This applies only to the Liege proof house around 1906. Later (post 1924) the service pressures were increased.
Nothing indicates a makers mark. Serial numbers do not indicate year or maker. The *N, etc are controller's marks, which tell us nothing during this period.
Judging by the sling swivels attached, this gun was originally used in Europe.
Pete
