The numbers on Kolar chokes refer to the amount of constriction compared to the nominal bore of the the barrel. Thus a # 20 choke on a .750 bore has a choke diameter of .730.
Kolar labels their chokes as follows:
15 - Lt. Modified
20 - Modified
25 - Improved Modified
30 - Light full
35 - Full
BTW, they note that their chokes should not be used with steel shot unless the choke is so marked. I have never seen such a label.
Ted K.