No, not cosmetic. The dual-action truss rod in the M-20 is functional. Unlike compression rods, which must be secured into the wood at one end, held in a hole at the other, and “pinned” in place in a bowed position beneath the fretboard, a dual-action truss rod is a fully independent machine. It sits comfortably in a slot in the neck, is not “fixed” to anything, and it is the builder’s choice whether to provide access to the adjusting nut on the headstock end or through the soundhole. Twist the nut one direction and the rod bows forward, twist the nut in the opposite direction and the rod bows backward. The dual-action truss rod lets you “force" the neck to bow in either direction, whereas the compression rod only permits you to “force" the neck to bow backward (you simply loosen the nut to let the neck bow forward again, and hope the neck hasn’t warped permanently in a back bowed position).
Compression rods have fallen out of favor, and dual-action truss rods have been all the rage for some time now. Many builders choose to access these truss rods through the soundhole for aesthetic purposes (they don't need to add a truss rod cover to the headstock). But it makes no difference to the guitar; adjustment via either soundhole or headstock works just fine.
“Vintage” has a certain appeal, no doubt. But there is also a cost involved, especially with regards to maintenance. As you say so well, "buying a vintage guitar might be more fun than actually owning one.” Of course, every now and then, someone here on the forum might add, “the only thing more fun than buying a vintage guitar is selling one!"