If I remember correctly, the Polytech's had inconsistanty/improperly heat treated receivers. Some had bolt issues. Replacement with a GI bolt corrected that part. There were (and perhaps still are) some shops that would strip down your receiver and properly heat treat it. I do not know if there was a serial number series that had more or less problems.
I'm sure there are a few folk more learned than I on these guns.
In life, strive to take the high road....It offers a better field of fire.
"Robo is right" Fuzzball