Fortunately for the average collector, the saving grace is found in post #41 of the link:
"...After all the time and money we spent on this one, you'd probably be looking at a $5/hour profit if you tried to pass it off as an original HP. So the payoff is simply not there on a gun like this, let alone a run of the mill P.38. The only guns I would be concerned with would be the really rare ones...."
But therein also lies the warning for collectors with an itch and more money than experience and learning. When a common model of gun increases in value by five or ten fold just because of some marking, it is an open invitation to fakery. Many unscrupulous folks don't have the talent or access to the four craftsman who joined their skills to produce this particular object lesson, but they don't need to in order to swindle people who can't resist "stealing" an underpriced "bargain", and who haven't done their homework. All the faker needs is a replica waffenamt stamp.
What is often said about "buying the gun, not the story" also applies to some extent to paying huge premiums for markings, or for contrived "rarity". Does the pitch that "they only made 20,000 of these" truly make it rare (and therefore intrinsically desirable)? IMO, not without more, and not without a lot of people who imagine that "rare" and "desirable" are synonyms.
LOL, I got a couple guns they only made less than 10,000 total of each including all the variants and I don't think that makes them particularly rare and/or desirable to anyone but me. MR9 and Mauser M2, in case you're wondering.