Technical Latest adventure (breakdown due to ignition timing shift) with 1947 Chevy Fleetmaster

Had a ball with the old car 2 days ago - a friend and I surprised another friend I'd hidden the car from for 2 years while I worked on it... Took old-timey pictures (sepia filter on camera) and used old shotguns and jugs as props. Drove the car about 40 miles or so this time. I was taking one friend home, and the thing just shut off like the ignition was switched off! Had gas in the filter, had spark when points would open, and it would nearly try to run after a few tries - but never started. In the end, I could tell the timing was off by about 30 degrees (retarded). I have an old distributor cap that I've used a 2-1/8" holesaw to bore the center out of - I put that on, and with the flywheel timing mark lined up on compression stroke of #1 cylinder, I could see the distributor rotor was between #4 and #1, when it should have been dead on #1. Distributor was not loose, gear pin was in place and gears were in good shape. Took some measurements and realized that it's possible to have the distributor a bit high instead of seated. When the distributor isn't fully inserted, any axial free play in the shaft can allow it to raise or lower, thereby changing the timing (because of the helical cut gears). My distributor has about 1/8" of axial (lengthwise) play between the bottom of the distributor body and the gear. If the shaft goes up, the timing will advance, and if it goes down, it will retard. I'm at a loss as how the shaft stayed up for 100+ miles, but it seems as though it finally dropped and retarded the timing. So I reseated the distributor and reset the timing. Now, I realize it's also possible that something maintaining the camshaft position could have fallen out - and the helical cut cam gears could have been responsible - but that would have also affected the valve timing, and since the engine runs OK, I think that can be ruled out. Only base I think isn't covered is the possibility the centrifugal advance (working before and after this incident) could have had something keeping it from fully retarding, and that something eventually fell out. I do know the original distributor cap had its carbon button fall apart - so it wasn't pushing the shaft down - but it ran about another 40 miles with the new one before things went bad. Anybody had a similar issue? If there's a message in the manual about making sure the distributor is fully inserted, I missed it. Since the vacuum advance clamps around the distributor shaft, it will hold it at whatever level you determine - intentionally or otherwise - and if it's dirty and all you do is rotate it, it's likely to stay high....