The nut itself is the compression fitting. It has a thin tapered front section that bites into the tubing and seals the first time it's tightened. Once the nut attaches itself to the tubing, it is supposed to stay there during later removal/replacement of the tubing in the oil connector. If the nut is installed properly the first time, about 1/8" of the tube or a little more protrudes past the front of the nut, then the tapered portion of the nut crimps itself to the tube as it's tightened..

Every local parts store used to stock those nuts, but that was 50-something years ago when these engines were daily drivers. Someone posted a reference with a part number recently but it was for a 1/4" OD tubing fitting, not the 3/16" OD one that fits the oil connector line.
Jerry


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