Years ago while working I found they were using a lot of SS fittings and were concerned about over tightening and galling.. these soft metal tapered rings that came in different size for different size A&N fittings. I was doing plumbing on my car and looked around and found they were available at Summit through a brand I can't remember. The ring fit over the male side and was crushed to seal putting the force on a small malleable ring. Ideal for the aluminum and SS A&N, JIC and so on. I bought several sets doing my
oil cooler fuel pumps and filters ,regulators an tank fittings. I'm having a time just figuring what to call them for a search. They were a soft metal tapered ring that were compressed in the fittings union. They could be reused if you had not gone nut in the first place.