You need to also find the bore size for each barrel. Subtracting the choke size from the bore size will give you points of choke. If for example you find 18.4 marked somewhere (on both barrels) that would give you 24 thou in one barrel & 32 thou in the other. Somewhere around 1/2 & 3/4 or Imp & Imp Mod.

The 17.6 & 17.8 are mm, likewise my imagined 18.4. You convert these figures to Thous of an Inch to arrive at the designated choke.