Mike,
If that is a late 1930's gun it could still be 2 9/16" chambers and the bore would be .662. Before that it was .650.
L.C. Smith gave percentages of a specific shot charge, a given powder charge and patterning at 40 yards with a 30" circle, and that is how they worked out chokes with percent of pellets at that range.
The best thing to do is try different shell loads, and different manufacturs because they will all be different.
If tyou have the chokes opened you might find it doesn't shoot any better or worse than before.
Light modified could be .015-.020 and improved modified could be .025-.030 contriction.
