The vacuum advance has had the same design for decades. Its actually comprised of two parts. The Octane Selector Plate/distributor clamp, and the vacuum advance canaster. The vacuum advance canasters are all about the same design, the Octane Selector Plate/clamp arrangement has a couple of different variations but all look and work the same. The port where the vacuum line enters into the canaster, if you look into the port, you will see that it is a flare arrangement like a wheel cylinder, or brake line distribution blocks. And they all (as far as I have seen) use standard 3/16 flared brake line. So if you have a compression fitting on yours, that was added to the system by a previous onwer. Frankly, its not all that uncommon to see these kinds of modifications. These motors were installed in everyday man's car and/or truck and in those days, every single dollar counted so if you could make repairs with what you had on hand, rather then pay a mechanic do it, then that is generally what you did. So it appears that a previous owner fixed the vacuum advance line with what he had on hand.
If you want to make this right, get it back to what the engineers originally specified for the advance system, remove the line and fittings, and make the installation as the engineers designed it. Direct install to the vacuum advance with the flared 3/16 line. At the other end of it, at the carb, there should be a fitting. I want to say 1/8 npt at one end (carb) and inverted flare for the line at the other.
