Harry,

Here are the Belgian proof marks:
http://damascus-barrels.com/Belgian_All_Proofmarks.html

The 1kg 273 is the weight of the barrels at the time of proof. It was used from 1892 to the present. After 1924 they no longer used a decimal point. So your barrels weighed 2.806 lbs at time of proof.

The chamber length is expressed in the proof marks. It will be in mm, so a 12-70 is 12ga 2 3/4".

Pete