How much the shot is constricted as it leaves the barrel.
Unknown.
Improved cylinder
# They relate to choke type, i.e., full, modified....
If you mean a choke bored barrel, it designates the amount of constriction a shotgun barrel has.
The marks on a Huglu shotgun choke typically indicate the constriction level of the choke tube, which affects shot dispersion. Common markings include "C" for cylinder, "S" for skeet, "IC" for improved cylinder, "M" for modified, "IM" for improved modified, and "F" for full. These markings help shooters select the appropriate choke for different shooting conditions and target types, influencing the spread and range of the shot. Additionally, some chokes may have numerical designations that further specify their constriction measurements.
4 notches = cylinder choke (open), and one notch = full choke (tight)
They probably denote what choke the barrel has.
It stands for Modified choke.
Degree of choke
Go here for a good explanation - http://hunting.about.com/od/shotgun/l/aastshotgunfaqa.htm
yes it is a choke marking. What you list doesn't follow the normal convention.