I've seen the double ring compression fittings from parker,but only in stainless steel for stainless steel tube. Never seen them in brass for copper tube. Those are a little pricey and I see other brazed swage joints so I'm not sure why you even used them?

Most people just buy the standard compresion fittings from home depot or any hardware store.

Let us know how they perform,both during the intial leak test and if they hold up over time. Vibration is a big concern for me.