"U.S.",
Your jug choke job could be quite old. "Retrofit" jug choke jobs date can back to the 1870's.
FWIW about jug chokes, based on my experiences with them:
1. Jug chokes can yield excellent patterns. 85%+ full choke patterns are not uncommon, especially in muzzle loaders.
2. Jug chokes quite often do not seem to require quite as much constriction as other kinds of chokes do. .020" constrictions can yield very dense, even full choke patterns.
3. Jug chokes very often do not "like" loads that feature shotcups. This may be due to the possibility that shotcups may counteract the chokes' "spread 'em and then squeeze 'em" action on shot charges.
4. Jug chokes may also not "like" slug and, especially, round ball loads. This may be due to the fact that the projectile loses some contact with the bore in the "jug" and then may not enter the choke's constriction in a consistent manner.
