Fittings of those type are used on copper tubing with a built-in sleeve which compresses into the tubing. I don't think those are reusable. If that fitting is brass, those are readily available. Here's one place Link


Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.