[DonVoigt]

John,
Are you "sure"? Do you have guidance from the ATF as detailed as what you wrote?

Suppose your Mauser luger example, has a DWM toggle; or one could argue that the receiver is the replacement part and not the barrel; since the frame is the "gun" anyway! Or just remove the 1941- there are no rules against removing the date!

What you write may seem logical, but we all know that logic does not always apply.
Is not the key here "original configuration"(as written by ATF ) and not original parts?

I do believe this is one of those don't ask the ATF questions; they would have no idea how to answer the question and the entire agency might melt down. Remember these are the guys that approved "bump stocks" and "14" shotguns" recently!