At one time there was a modification in the distributor vacuum for 1966 models in California for smog. It was some type of vacuum bleeder or something like that. I've still got one somewhere. When the state no longer required smog tests on 73 and older car, all that junk went away. That may be what you have.
Jim
There were many systems retrofitted to cars required (at one point or another) by the state (CA) one such that relates to the solid dist advance line was the KarKit. About $15, included two vacuum plugs and a decal. You cut the steel line, cap (well you were suppose to) each end and slap the sticker on the face of the speedo.
Last car I had to have smogged I had to put one on, It was a 67 GT350 and had to install a dist with vacuum advance so that I could plug it off. Guess I might do the same today if ever caught by a sniffer or road side test station. Last time I saw one of those was about 6 months ago and they travel with a simple dyno on a trailer they tow
Sorry for getting off thread a bit