There are a number of general marks that were used to denote that items were made by Native American Indians, usually Navajo. These include
IHMSS which stands for Indian Hand Made Sterling Silver
IHMS which stands for Indian Hand Made Silver (so would refer to items made with coin silver or other silver of a lesser silver content than sterling silver)
HAND MADE BY INDIANS – per guru Kim Messier, this stamp was only used at the Garden of the Gods Trading Post.
More generic HAND MADE stamps were used at shops or collectives where a number of NA artists worked, some doing silversmithing, some preparing stones, some setting stones etc. This would include stamps used at White Hogan.
I’ve also seen HMIJ which stands for Hand Made Indian Jewelry
All of these designations were intended to identify the items as Native American made. Although they often appear alone, I’ve also seen most of these stamps along with an artist’s name or hallmark to designate the particular artist who made the piece.
I hope this helps but the bottom line is that you have a very handsome ring !