A mechanical advance is just that, mechanical. It uses weights inside to advance the distributor spark timing. The more the motor revs, the more advance you get. Use different weights to control when and how much total advance you get.
A vacuum advance has the mechanical weight plate inside of it and also has a vacuum control on the outside. The vacuum is connected to the intake manifold or carb and when the motor is under load and pulling vacuum, it will also advance your timing.
Your '64 would have come with the vacuum type distributor. IMO they are better as they use engine load to set the advance versus the mechanical only that just relies on the rpm of the motor.