Choke tubes can be identified by the number and placement of small notches found on their muzzle ends. Be sure to clear and make safe any shotgun before looking at any installed chokes.
Notches / Choke Constriction
O / Extra Full
I / Full
II / Improved Modified
III / Modified
IIII / Improved Cylinder
IIIII / Skeet
IIIIII / Cylinder
I-I / SC I
II-II / SC II