The numbers you found under the forearm indicate various barrel measurements .
18.6/70---This is the chamber diameter, 18.6mm which equals .0632", the diameter of a 16 guage chamber. 70 is 70mm chamber length.
16.5/17.1---16.5 is the diameter of the choked portion of the barrel-16.5mm which equals 0.649". 17.1 is the diameter of the 16 gauge bore-17.1mm 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.