This is a nice old 12-fret D12-20 that I recently was traded. It's a player's guitar for sure, as it had some funky alterations to its bridge and I had to remove a giant moon-shaped pickguard and buff out the crud from under it to get it at least presentable.
This was Martin's first official 12-string model (essentially ended production 10 years after its introduction in 1964(?) I think) and it shared the original (Ditson) dreadnought body shape with a longer upper bout and round corners, more centrally-located bridge, and 12-fret neck join. It's essentially the same guitar as the "original dreadnoughts", save slightly heavier bracing and a larger slotted headstock to accommodate the tuners.
After setup, cleaning, light fret dress, etc. this guitar plays really really nicely and has that big sound you'd expect of a Martin, made rounder and fuller by the 12th-fret join. 1 7/8" nut just like the original dreadnoughts... though for a real purpose here, as it gives perfect spacing for the extra strings.
Build is similar to a D-18 (mahogany b&s, spruce top, hog neck, rosewood board and bridge, etc.) save that there's an extra white purfling line around inline from the binding and there's a D-28 style (I think?) backstrip inlaid on the rear.
New pickguard. Note strange bridge: I've filled holes (still have to touch them up just a bit) behind the main holes because the bridge was shaved really low so there's not enough angle from the rear holes to get over the saddle at the proper angle. I've cut extra string ramps from the main holes and slotted the saddle so that I can load all 12 strings from the 6 first holes... this is similar to how old 1920s Stella 12-strings with pin bridges were loaded. Looks sharp, too. Also note how someone installed a whole new section of bridge in front of the (original) bone saddle. Peculiar.
A looker, yessir.
And original tuners are working just fine. They're quite tarnished, however.