Ithaca Flues.....

[Quail1]

Austin,
I don't believe the stamping had to do with gauge. I believe it was different time periods of manufacture.
I have only 20 and 28 gauge guns so I can't speak to the other gauges. Some of my guns have no choke stamping, others have just one digit 1-4, and then some are in fractions, 0/4,- 4/4.
These are the only pictures that I have with me.
I am finishing the last weekend of turkey season, but will post photos when i return home.

[Eldon Goddard]

In my younger and more formative years I paid a gunsmith 50 bucks to tell me what was stamped on the bottom of my barrels, learned my lesson and bought a stan baker bore gauge. Anyways I looked at all my flues and came up with the following
20 ga twist barrels SN183298 not marked
16 ga chine Damascus SN303172 marked 2 and 4
12 ga smokeless powder steel SN343020 marked 0 and 4