From what I have heard, they are the exact same mic, except for the whole cable thing. The "M" does stand for Mogami. The inscription on the bottom says: "V67 Mogami" - not v67M. Very interesting indeed, which got me thinking.
Being a marketing person, this is my spin on this whole G vs. M deal. Marshall Electronics is the distributor for Mogami cables. Up untill now, their core business has been that and surveilance equipment, of all things. From a marketing standpoint, I'm sure you can guess the challenges they face trying to carve a niche as a MICROPHONE company.
Now aside from the whole surveilance equip. thing, think of all the the less-educated people out there in the consumer market that associate them automatically with the other marshall (guitar amps). To make matters worse, you have a mic with the name "Mogami" jumping right out at you on the front of it, clear as day. "Mogami? Aren't those the guys who make the cables?" The Mogami thing, in my opinion, kind of cheapens the look of the mic. Almost like you bought a bunch of cables and got a cool mic with it.
To make matters worse yet, People are confused at the differences between the various models - HENCE THE SUBJECT OF THIS THREAD which only further complicates things for them in their quest to establish their brand identity.
Meanwhile, the Marshall name is starting to pick up momentum in the budget and semi-pro market for large-diaphragm condensers. The word is out, and the V67 is generating quite a buzz. And as with the Studio Projects C1, it has all taken them quite by surprize. Their 2001P was supposed to be their darling, but the reviews kinda' sucked, for lack of a better word. (And another interesting Studio Projects parallel, the 2001P was also essetially the same mic as the Joemeek 47, which SP was originally hoping to be THEIR darling) . The 2003 was supposed to succeed that, but the public became surprizingly enamored of the V67's transformer-ized sound and unusual looks.
Now is not the time to dampen the buzz with a product that is poorly marketed.
So Marshall figures it's time for a blowout on the "black sheep model." And my guess is it should help them out twofold: By ridding themselves of the association with "cables" and establishing, once and for all, the name and identity of the mic. The V67Gs will remain, I'm sure, as it carries a more distinctive look you're not soon to forget, and doesn't have the Mogami name stamped all over it like a cheap billboard.
The marketing department's blunder is the consumers' gain in this instance. Think of it as an early Christmas present.