What is the meaning of 'sts' on the inside of a ring?

The abbreviation "sts" on the inside of a ring stands for "sterling silver."

It is not my understanding that this is an approved stamp for sterling silver. Jewelry that is sterling silver can be marked sterling ster. or 925 and up to show the content of the sterling per other metals. STS is a jewelry company that has a New york, Canadian, and foreign base. So likely the ring is a maker's mark.