Why Were Carbines Factory Parkerized When ALL USGI Thompsons' Were Factory Blued?

[BrianQ]

Other than the Inland's, how would one determine the factory Parkerizing from the subsequent arsenal rebuild Parkerizing, especially if the Parker-lubrite was applied over existing scratches?

mpd1978 hit on a few ways. Those plus knowing how the individual manufacturers did thing helps As does examining known original examples.