The moderators were definitely removable -screw on, screw off - but if someone got the moderators registered, I can't see how they'd neglect to also register the gun. I mean, I'm sure they could have been seperated, but there's a lot of stuff in the registry that predates Vietnam, so I'd think someone must have had the good sense to get some of the guns in and registered along with the moderators. Even if no one else has one, I'd bet you'll find at least one in Reed Knight's collection.
On the one tax MG & supressor combos...
You are correct thatthere are some one-tax machinegun and supresor combinations out there, but generally BATF/NFA didn't allow them, and required wach device to be seperately registered. I know some are on the registry and transfer without any hassles, but the way I understand it, once BATF/NFA got wind of how people were registering MG & integrally supressed guns as one unit, BATF started disallowing it. As far as I'm aware, there were only a handful of MP5SD's that were actually done this way. I may be completely off base on this, but I seem to recall they were Ciener conversions.