Looking at 12 gauge constrictions from this Ithaca chart; full (4) = .036", IM (3) = .027", Mod (2) = .018", IC (1) = .009" & Cyl = 0.000 (0) Note the numbers in parenthesis are the old Ithaca numerical choke markings.

Doesn't look that much different to Browning Specs to me & again the IC choke is equal to British & Continental ¼ choke. There, these chokes would be called Full, 3/4, ½, ¼ & Cyl. Note that 3/4 of .036 = .027, ½ of .036 = .018 & ¼ of .036 = .009 which matches exactly with the listed choke diameters. IC other than in the US generally meant from 003" to .006" of constriction. It was meant to improve the performance of a cylinder bore without materially reducing its spread, just help even out the pattern. It was a great improvement with the old card & felt wadding of the day.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra