Mark,
After doing a little research I have found a little info for you.
These ring are termed as " Officer's Totenkopf Rings" and "Kantine rings".
You can do a web search for "Officer's Totenkopf Rings" and get numerous pages to research.
"Silver Totenkopf rings like this were available for private purchase only and were often referred to as "Kantine" rings. They often appeared in ads in the classified sections of the popular German war-time periodicals like "SIGNAL" and "DAS SCHWARZE KORPS" so it's actually a very well known and a well publicized design. They were actually considered "unofficial" and were offered for sale to all Waffen-SS soldiers regardless of their rank, although it's assumed only officers wore the silver versions while EM/NCOs purchased the cheaper silver plated or brass versions. Even though the silver versions were relatively inexpensive at the time, they were considered a luxury item that most EM/NCO SS personnel either couldn't afford to purchase, or were simply not "zealous" enough about their service in the Waffen-SS to spend their meager soldier's pay to own one."
Here is one website I found that has numerous rings for sale from about $300 and up (authenticated rings sell for a much higher price, such as the rings on this website, unauthenticated rings seem to be selling for under $100), though not one exactly like yours, there are two pages, be sure to check out the second page, you maybe able to contact the person from this website to get more info or to authenticate your ring.
The rings are about halfway down on the first page and at the bottom of the second page.
Items page.
http://65.160.172.250/ssi.html
Home page.
http://65.160.172.250/main.shtml
Also there is a book written about these rings that is highly touted by collectors titled Craig Gottlieb "SS Totenkopf Ring", you may want to see if you can check out this book at a library or purchase it for this complete history of these rings.
If you buy this book do a search for the best price.
raiggottlieb.com/data/inspect.asp?Item=2880&Filter=Book+Store&Name=The+SS+Totenkopf+Ring,+Standard+Edition
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