How could they not at least polarize the plug or put some kind of grounding in the plug??
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Mitch, the plug is probably polarized although they didn't use the wide-blade-narrow-blade convention in those days. If you look at the cord itself (I assume it's zip cord) you will see that one side of the cord has ridges on it and the other side is smooth. The plug blade that corresponds to the ridged side of the cord is neutral and should go into the wide slot of the outlet.
However, I'm not sure plug orientation, or the lack of a grounded AC cord is what's causing the "thump" you describe. That sounds more like static discharge built up as you walk across the carpet and going to ground through your rack or equipment when you touch it. And the "thump" is the RFI created by the discharge.
Achieving noise-free ground pathways through your system is a lot of science and sometimes a little art (when the science doesn't get you all the way there ;--)
My system is dead quiet, but in winter, when the humidity is low, if I touch the tonearm before first touching something that's grounded (like a preamp knob) I get a huge "snap" through my speakers.