[Henry Schmidt]

It sounds like the shop used the original pistons supplied with the engine before rebuild.
If there was no noise (sometimes new engine owner hear noises they didn't hear before) prior to the rebuild, I would suspect an intermediate shaft knock.
Very common on early SC engines.
It's possible they replaced the gear. "-1" gears are harder to find. Perhaps they replaced a "-1" gear with a "O". This will create a knocking noise that is annoying but not a death rattle.