Interesting. Learn something everyday.
Yea, its one of those "gun family" things designed to limit changes. Otherwise, some smart a$$ out there would take a Mac-10 receiver, flatten it into a press, and then use it as an M-240b sideplate.
There were guys pulling stuff sort of like this a few years ago, "washing" paperwork by transferring guns several times with slightly different names each time, until the new gun wasn't the old one. They just kept the serial and make, and then made a "fake" gun using the information from the now-destroyed one. So an Mac 10 would become an M10 would become a M1910 would become a Maxim M1910, etc.
Click here for the indictment
It made the paperwork process a real headache for all of us for a while, as the ATF began rejecting forms that didn't 100% match the prior paperwork. My old PPSH-41 was apparently registered in 9mm when it was remanufactured, I guess because 7.62 tok ammo was hard to find back then. When I acquired it, the gun was in the proper caliber, and the paperwork matched that caliber, but my forms were rejected when I went to sell it. They went deep into it's paperwork trail and caught the change, and said it was a no go, they didn't care what caliber it was actually in. So I had to change my outgoing Form 4 to state that my PPSH was chambered in 9mm, otherwise the forms wouldn't go through. I'm wanting to say that my M-2 carbine had a similar problem, but my memory has hazed with time. I think when it was registered, the guy just put what length he thought that the barrel was, within an inch or so, and called it a day. By the time I got it, the paperwork stated the exact barrel length, to like the quarter inch. Got rejected. Yea...