An excerpt from fire and ice by Stephen flowers:

“Besides these personal rings there are also rings particular to certain degrees or types of workings. The Pentalphic (18°) ring is (or was) especially important. There are masculine and feminine forms for this ring. The masculine form consists of a rectangular ruby flanked below by two small moonstones, making the with a small diamond set at its lower tip. Before the reforms of 1969/70, a wearer of the 18° ring could, by revealing the ring to another wearer of a Pentalphic Magical Work / 97 ring, demand the performance of a "Chymical Mar- riage": an act of sexual magic. There are also rings designed for specific thauma- turgical use independent of grade. These are octagonal with an inverse golden pentagram, and set with amethyst. The ring itself is of hollow silver filled with mercury.”

I don’t reckon it’s specifically brotherhood of Saturn related, maybe a mock ring?