some of the older Hoboken M-20s and F20s have one-piece backs, and no center brace going north/south on the inside of the back. many, or most, have the 2-piece back, along with the center brace.
Not sure about the new Oxnard crop. it does appear from photos they are one-piece like you are observing. I'll check one out again in Sam Ash soon.
Those are nice guitars. Oxnard made Guilds are still working on an identity of their own. Still earning the respect of the hard-cores, and working out some growing pains and kinks. But those M-20s are nice players and the tone of the M-20 is there. Apparently they did not do a good job of disclosing the construction methods on this maiden voyage for Oxnard model, and the set neck is actually also bolted on the inside. But i've played this guitar, and it is a nice M-20 that really does nail the sound. the 1 3/4" nut width is a debut for this model and is a nice option to now have in the mix of M-20s. If they have one-piece backs, im not sure "which variation of mahogany" that might be, and it doesn't really change the tone but that is definitely super cool if that is the case, and I thank you for pointing that out!
Remember the M-20 was always a cheap, student, beginner, even child's instrument, that sounded incredible for what it was. They have also been using the same, or similar, "variations of mahogany" since back in the 60s-70s, that they do today. I think the current M-20 is solid for what it is, definitely gets the job done. There have been some reports of duds or blems but all guitars have that from all guitar makers.