The distributor
must move when vacuum is applied to the vacuum advance.
If it doesn't either the distributor is tight in the block(O ring used on housing is one example of a problem), or the advance diagram has a hole in it.
There were a number of vacuum advance units produced by Chevrolet through the years.
An obvious difference was the mounting angle of some to clear the side mounts(passenger car only).
There were also different advance rate for different applications(controlled by the internal spring rate).
Distributor part numbers by year.
