The numbers you found under the forearm indicate various barrel measurements .
18.6/70This is the chamber diameter, 18.6mm which equals .0632", the diameter of a 16 guage chamber. 70 is 70mm chamber length.
16.5/17.116.5 is the diameter of the choked portion of the barrel16.5mm which equals 0.649". 17.1 is the diameter of the 16 gauge bore17.1mm which equals 0.673". The degree of choke is 0.6mm or 0.024". This equates to an improved modified choke.
The 3 stars is the European marking for improved modified choke.
I do not know what K and 0.775 mean
34 Grams is the weight of the shot charge which is 1.2 ounces in US terminology, a moderately heavy shot charge.
Your shotguns should present no problem when used with standard 2 3/4" or 70mm ammunition.
