Do you have this ring in hand? If so, you're in a better position to make out the marque on the back than we are - could it be "Made in Mexico" by any chance? Or maybe "Made in Taxco"? The motif on the front appears to be an eagle wresting with a snake, which would be consistent with a ring of Mexican origin.
Here's one version of the eagle and snake from Mexico's coat of arms:
Although there are a few sites dealing with Mexican silver on the internet, they all seem to draw from the same limited sources. I've had better luck with books available in the library. Here are a few sites to get you started: