I've not seen as many examples as some folks have, but I can make some observations.
1) the Norinco's with the "00###X" serial numbers were among the first Marstar and Lever imported. They were assembled in 2000, though appear to have been imported in 2005-2006 or so. They are 1980's manufacture near as I can tell for the US market.
2) Later Norincos were marked with the import year separately. but were probably made around the same time.
3) The Bell/Polytechs I have seen had a rougher finish where the import markings are. I believe, though cannot prove, these receivers were likely also marked identically to the Norinco receivers but had those markings scrubbed and the Bell markings added. On the few examples I've seen there are clear signs of filing or grinding below the import marks, while the rest of the receiver(s) are much better finished.
Finally, for some reason, some of the Polytechs are finished in a dark almost black park that comes off too easily, while others have Norinco-type dark grey. I assume two or more different assembly batches are to blame.
Finally, the recent guns from Norinco and Poly seem to have engraved serial numbers on most major components despite the ugly dot-matrix import stamp on the poly's and electro-pencilled markings on the Norincos. Presumably the guns were serialized separately from when the import marks were applied.