Slant 6 engine ignition timing stays retarded.
I'll look for that second slot though I can't imagine what it would look like. Can you be more specific? I've had this dist out many times in the last several months and don't remember seeing something like that.
Turn distributor upside down so the drive pinion is pointing toward you. Notice how the distributor hold-down plate is held to the underside of the distributor body with a bolt. Notice how that bolt goes through a slot rather than a round hole. There you are.
Also, if you're getting significant vacuum advance at idle, your settings aren't where they should be, but that will likely self-correct once the base ignition timing is where it should be; because it is too retarded, you will have had to open the throttle wider to achieve the 750 rpm idle speed, which will have uncovered the vacuum advance port.
Spec in '73 is TDC, but you'll have a much better-running car, without creating new problems, if you'll use the 1960s spec of 2.5° BTDC. Even better still if you will bypass the OSAC valve (right side of firewall on the engine side) and just run the hose directly from the carburetor to the distributor.
Also-also, make sure both of the distributor's O-rings are present and in good condition; info
here.
Also-also-also, tune-up parts and technique suggestions are in
this post and Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted
here for free download.