The design y'all are talking about and that Tim posted links/pictures of is a compression sleeve and tube nut "all in one" piece. The end of the tube nut will crimp/compress onto the tube when it is tightened in the fitting. The tube nut is then not reusable on another line.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-