In many cases the definition of 'Full' and 'Extra Full' have changed. A lot of the older guns that are marked 'Full' will mic at .035" or even .040" of constriction where today .030" is pretty much standard for a 'Full'. So, even though they weren't marked as such, a lot of older guns are choked 'Extra Full' and 'Full'.
More than likely the the model 21 that you have was ordered to shoot live birds as well as trap. Just a guess, it is after all shooters preference some preach IM some M I prefer F others prefer XF. Just a matter of what you like.
According to Dr Jones' book, hard shot and fiber wads produce tighter patterns through the same choke than plastic wads and medium or soft shot. If the old shells contained hard shot they didn't need so much choke to throw a tight pattern.
EuroJoe is right. But, that was the definition of a Full choke when hulls were paper and so was the wad. A .040 constriction would give you the percentage back then. Also back then IM needed .030 to get the percentage. That is now considered Lt Full.
I think that the US should just adopt the .005 increment constriction table as the US or American standard of choke. Shooters need to shoot some patterns with their ammo of choice to find out what kind of pattern their gun and ammo shoots.
I should have been clearer about why I posted the scale. A lot of older guns are choked tight by todays standards, but normal for the standards of the time of production.
Depending on your POV todays guns could be considered lightly choked or in the same sense older guns could be considered over choked. It depends upon which set of standards you were raised on.
I have a set of older fixed choke Beretta barrels thaty are marked IM/F. If you measure them out with a bore mic the bottom barrel is .029" and the top is .038". I would consider these barrels to be LF/XF to the set of standards I was brought up with.
It's a potato/potato thing. They are all the same just expressed differently.
short shucker if true bore(a barrel that is standard 12GA bore .729 full choke should be .035 or .694.
So .038 can't really be X-full. The Barrels mentioned in the OP are true bore barrels. As I don't believe they were backboring barrels in the time frame of the model 21 trap.
I understand that the numbers that are in the modern shotguns of today are anything but standard. The term Full choke is still .035 over bore It doesn't matter whether the .729 or 7.50 the constriction is still .035. This is one of the reasons that the shotguns of today are more popular as over bored barrels seem to be able to be custom tuned to the bores.
I am just talking about the shotgun in the era or time frame that Hammer was referring to.