Upper receiver lapping?

[LRRPF52]

I believe most uppers are cast and not lapped before assembly for mass production guns or complete uppers that most companies offer. I would suggest canceling that item and ordering it through another vender that has it on the shelf. Amazon has them and you can have it in a couple of days

Upper receivers are either forged and finish machined, or machined from billet aluminum, then anodized.

The boring and face-cutting operation is done so that there should be no way for the face to be out of square....however, once anodizing is done, it can create a face that is no longer square due to non-uniformity in the anodizing process (a common headache in the anodizing process).