Ok, so I showed the ring to a jeweler yesterday and here is what they said. The ring dates to the 1950s. If it was foreign-made (i.e. Mexico), that would appear on the inside of the band (due to a law passed in 1929). They said the style doesn't look Native American. Rather, given just initials for a maker's mark, the ring was probably made by an individual jeweler or a private company - which is why the maker hasn't been IDed. They said it was also probably made with a rolling mill.
With this update I'll mark this thread solved. Thanks again Mike for the assist!