distributor hold-down prevents timing adjustment?

The outer ring on the crank pulley is a damper ring for internal crank vibrations; that outer ring is held onto the hub with a thin layer of rubber (which provides the damping). That rubber can get old and crack and let the outer ring slip, which moves the timing mark off of where it should be.

Eventually, that rubber may fail and let the outer pulley ring come off... or fly off. So if the new mark is some degrees away from the old one, then it may be time to start thinking about a pulley/damper replacement. That requires specific puller types for both removal and installation.

And it could be that you just did not find the right mark to use before.

Refer back to post #14; that is correct about the timing chain wear and retarded timing; the timing retard can be adjusted out for ignition timing by moving the distributor, but the cam timing cannot be adjusted out. Retarded cam timing of 5* will make the performance quite sluggish compared to what it should be.