UPDATE: I received the Harley Benton (HB) 6-on-a-plate tuners from Thomann GmbH today (brass gears/larger buttons, in the photos, below). The plate is nicely etched in an attractive pattern. The gears are brass, and the plate, posts, bushings and screws appear to be bright zinc-plated.
The posts nearly center on the three (3) Guild 12 string headstocks I tested (including a 1975 F-212). To be fair, the original tuners do NOT center perfectly, either.
The screw holes on the plate of the HB tuners are spaced identical to the original screw holes on the back of the headstock, however, to align the posts as close to perfectly centered as possible in the existing post holes the existing screw holes would need to be filled and re-drilled, as the entire set of holes on the tuners are shifted toward the end of the headstock by approximately 3/32" (2.5mm).
The bushings are loose in the holes and would either need to be replaced (using either the originals or any number of replacements) or glued in place. A single HB tuner plate (one side's worth of six (6) tuners) weighs 3.72 oz (106g), where the original tuners weigh 4.74 oz (134.5g). Unlike the original tuners, I could not get all the posts to sit perfectly snug against the plate by tightening the screws holding the gears in place (there was still some wiggling on four (4) of twelve (12) posts - I believe the screws are bottoming out on those four (4) posts, which could be remedied). The particular tuners I ordered have a different button style. Overall, I believe the Harley Benton 6-on-a-plate tuners are a viable replacement tuner for Guild 12 strings.