I'm not even close to an expert on this, but maybe Union Switch used a different finish than NPM did.
What block of serial numbers is it? Perhaps the finish was changed at some point during the war.
Does your stock have any re-arsenal stamps on it?
Honestly, of all the carbines I've seen, they were all parkerized.

Also, purely out of curiosity, does it have a Trimble stock and handguard?