" but no distributor that I am aware of was designed to operate such that the timing was retarded when the engine was accelerated."
There were a few systems that had dual diaphragms, but in most pre-emissions systems, vacuum was used to advance the timing. Under acceleration, there is less vacuum, therefore less vacuum advance. If the first step in setting the timing is to remove and plug the vacuum advance line, then it is not a ported vacuum system.