I don't think there was ever an exact constriction which was considered, each maker being able to use their own standard as to exact configuration of the choke to produce desired patterns.
Old European guns including British ones generally were choked as follows
Full choke = .040"/1mm constriction
3/4 choke = .030"/.75mm "
1/2 choke = .020"/.5mm "
1/4 choke = .010"/.25mm "
I/C choke = .003"-.006" "
Cyl bore = .000"-.003" "
Older US guns generally followed these basic chokes with 3/4 choke bbls maked Imp Mod & 1/2 choke bbls marked Mod. Very few US makers ever seemed to bore a true I/C choke being much more prone to bore them to the 1/4 choke specs & label them Imp Cyl.
These dimensions seem to have been generally in use until the advent of the one piece plastic shotcup wads.
Many variations will be found to these general guidelines.
