Mercury used to sell them under the name "Safe-Jac"
It was mostly OPC'rs who I saw wearing them, including all the Team Mercury boys of the day. They were worn under the Gentex jacket........I don't remember seeing anyone wear one after Simpson and Lifeline first got into the life jacket game. I remember some of the Inboard guys having them as well......All with Gentex jackets.
I remember reading somewhere they were originally designed to be worn outside a snowmobile suit or winter gear. Most of the outer area had sewn on nylon (plastic? teflon?) panels that were meant to help deflect sticks and bush a rider might encounter while on a sled. I don't believe I remember seeing anywhere it had or added impact resistance.........But for awhile, it seems everyone had one!