non standard metal thickness but the Chinese, and they only did it as a solution to the problem of improper heat treating.
I tend to doubt this. There are other things more robust about the Chinese AK's: Barrel is thicker/heavier, bolt carrier is better machined; bolt has sprung firing pin (not floating); finish is generally better; trigger components are better machined/finished as well. I've done a side-by-side comparison of a stripped Polytech stamped AK and a Polish AK with US 1.5mm receiver.