How old is this clear (mason fruit?) jar with empty keystone
Trying to date this jar. Initial research says the empty keystone is for Rosso who was a wholesaler and not a manufacturer. This particular keystone seems to be tied to mason fruit jars as well, although I don't know if Mason would be a general term or a brand here. It has a horizontal diamond pattern design around the top & bottom of the body of the jar. The bottom of the jar is marked: (top line) JM 4616; (center line) 7, empty keystone. Any idea what the JM or numbers mean and what period this is from? Thanks!
The empty keystone is connected with Knox Bottle Company (and their series of glass plants). The jar has absolutely nothing to do with Rosso, who distributes reproduction fancy-colored tableware and novelties (not utilitarian jars) in EAPG (Early American Pattern Glass) patterns. Rosso uses an "R in a keystone" as his mark.
Just a guess, but the jar looks to be from the 1930s or 1940s. The JM-4616 is probably a style/catalog/inventory number assigned to that mold design.