Improved cylinder
Unknown.
# 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.
4 notches = cylinder choke (open), and one notch = full choke (tight)
It stands for Modified choke.
They probably denote what choke the barrel has.
Degree of choke
Inspector marks, choke marks, general information.
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.
They are a code used to denote choke size.