You can "see" the actual shaft vertical play by looking at the bottom where the gear/ spacer / etc is. It all moves up and down against the bottom of the distributor. That is not nearly as important as side play
Remember that this is a geared distributor with a very small diameter gear, so end play translates into timing changes, unlike the small block distributors.
But, the symptoms, with timing changes not effecting the pinging/knocking, don't point to distributor timing variations as the likely problem.
