The original compression ferrule was a cone-shaped piece, with the tapered end pointing toward the fitting that screws into the block. The fitting that slips over the tubing has a male thread. It's designed for one-time use, as it crimps itself into the tubing and cannot be removed again. A conventional compression fitting uses a barrel-shaped ferrule and a taller threaded portion that the female-threaded nut screws onto. The standard fitting also has a shoulder that prevents the tubing from sliding all the way through it. I have converted those to a slip-fit item by drilling away the shoulder at times, but the nut always stands taller than the male-threaded version. The same type of ferrule and male thread nut arrangement is used on most direct-reading oil pressure gauges.
Jerry
