Could it be that the spare parts are being turned into complete weapons?
What would make you more money as a DPMS CEO, selling all of your extra firing pins or saving them to put into your upper receivers coupled to your lowers? You not only fill the nearly unlimited demand for complete rifles but you turn a larger profit.
No doubt the various assemblers of AR type rifles are a close knit bunch and are swapping, selling, and trading among themselves. By design all the parts are the same, or nearly so. The difference mostly lies in the attention to detail in final assembly.
I just don't see any Gov't entity moving fast enough for this scenario to be true.