Hey Bass,
Nice mug base Hutch. I'm surprised to see that blank plate. I've found but 2 blank plate straight sided ones.
I'm betting on Root. This would have been one of their earliest bottles.
"R.G.CO……………..Root Glass Company, Terre Haute, Indiana (1901-1932). Mark used by Root in the early years (1901-c.1909). Toulouse stated these initials stood for Renton Glass Company, Renton, Washington (1907-1911). However, the “R.G.CO.” mark which was used by Renton likely appears only on certain bottles from the West Coast. I do not believe any of the midwestern bottles seen with the “R.G.CO.” marking originate from Renton. See “ROOT” entry...
ROOT……………….Root Glass Company, Terre Haute, Indiana (1901-1932). Mark is often embossed very lightly along the lower heel, and sometimes is almost illegible. Root produced a huge variety of soda, mineral water and beer bottles. (Root Glass Company is famous for having produced the first “Hobbleskirt” shaped Coke bottles, circa 1915 or 1916). There were two separate factories in the early years, one for fruit jars and the other for bottles. The fruit jar factory was purchased by Ball Bros. in 1908, and operated by Ball until it was closed in 1912 or 1913. Root Glass Co. was bought by Owens-Illinois Glass Company in 1932. Later the plant became part of the American-Wheaton Glass Corporation, in 1962 it was sold to the American Can Company, and then sold again to Midland Glass Company in 1968. After being purchased by the Anchor Glass Container Corporation, the property was closed down in 1984."
GLASS MANUFACTURERS' MARKS