I've never miked a 20 or 16ga standard Sterlingworth that had less than 10 thou factory choke. Even the 26" guns tend to have 10+ and 20+, or a minimum of what would have constituted IC/Mod when new.

I haven't miked a 30" 20ga SW, but the several 30" A grades were all full and fuller. Whereas 24 thou is accepted as Full in a modern 20, I've seen 36 in the older ones.