This set consists of a stainless steel collar ring (nut) and a male deck ring for cowl ventilators. They are available in 3", 4" and 5" sizes. The key to tighten the collar ring is included (inserts into the two small holes on the upper surface of the collar).
This set might be purchased to replace a damaged cowl vent component, or to use the collar ring to secure a cover plate on the dorade box when the cowl is removed (and you want to use a spare ring instead of dissembling the ring on the cowl).
We know, you want to be able to purchase just the collar ring since you already have a deck ring. Unfortunately, the collar ring is not sold separately.