>Why have I seen so many unmarked reactivated DEWAT guns?<
How do yiou know they were reactivated former registered DEWATs? Aside from that, many people who did form 1 reactivations as as well as CIIs didn't know about the marking requirement. Some deliberately did not mark the guns because it is an illogical and meaningless requirement. Other reasons.
>Was it just something that wasn't really enforced?<
Enforcement is difficult. Many registered DEWATs were transferred by F4 rather than F5, which is the proper paper trail, and were reactivated by someone along the way, so there would not be any evidence that the correct and legal procedures had not been followed. For a live vintage MG, if the original registration noted that the gun was rendered "inoperable", and a thorough researcher did his work, he could follow the trail of transfers and determine of that specific serial number had been properly reactivated. This deep level of scrutiny by any person of authority will probably never happen as the violation is so low on the priority scale as to be invisible.
A registered DEWAT that transfers to an individual on tax exempt F5 and is reactivated without paying the "making" tax or return transfer tax from the CII, the owner could be charged with "tax evasion" assuming that he was already in the clutches of ATF for some reason. But, again, if the person is already in trouble, with ATF this issue is immaterial. There have been a variety of prosecutions for "tax evasion" against various people for manipulating the transfer system to avoid paying the transfer tax, but I've never heard of anyone being pursued for the DEWAT issue.
>I own a couple. You've worked on two of them actually. No marks at all. Or perhaps I'm misunderstanding what exactly a reactivated DEWAT gun is?<
Yes, but it is a non-problem. A registered DEWAT actually has the same legal status as a live MG, in that it has a complete receiver, which is why they were required to be registered during the '68 amnesty. The tax is not paid for transfer of an inoperable MG if transferred by F5, a concession in the GCA'68 regs that is mysterious to me. Anyway, the reactivation protocol includes paying the tax one way or another. So, a reactivated DEWAT is just a live MG that has had the tax paid on it so can be properly transferred by taxed F4.
>Could a gun with a simple plug in it's barrel have already been registered as a regular machine gun maybe by someone who brought it back, so that when the barrel was cleared, the manufacturer itself never changed?<
OK, once again, the manufacturer/importer identified on the original registration NEVER changes, except for revision due to provable error. This info often got changed as transfers occurred for many different reasons, but it is supposed to be listed as it is on the original registration. Many "DEWATs" had poorly done or even sometimes nonexistent obstructions, welds, or other ways to disable the action or barrel. The mfg/imp info has nothing to do with the condition of the MG.
>Or maybe who imported it as a live gun?<
Depending on who imported the MG and their knowledge or competence, the info on the original registration can be almost anything. Also depends on what form was used for original registration. Until the end of the '68 Amnesty, anyone could import a live MG.
>But then it would have needed to be marked.<
There was no marking requirement prior to the GCA'68.
>Or maybe an amnesty gun?<
The '68 Amnesty was merely a registration scheme and had nothing to do with the condition of the MG, except that if an MG had a complete receiver, then registration was required. There was no such thing as a sideplate, tube, conversion part, kit, sear, combination of parts, etc, etc.