The info Cab furnished jibed with what I'd gleaned over the years. It seemed the older guys, the chev centric guys, all asserted manifold vacuum was the "correct" way to hook up vacuum advance to the point they'd arbitrarily change it to manifold on whatever car they were working on even if it didn't come that way. By the time I started my mechanic years (1973) mid '60's cars coming into the shop was a bit of a rarity, let alone earlier ones. At that time cars 5~10 years old made up the bulk of junkyard denizens which made parts for my '68 readily available cheaply.