Nice to see all those nice D46s. Mine is #DL100319, made in '81 or '82, can't remember right offhand and I'm too lazy to walk upstairs and look.
As for the 3-piece back, I heard it explained like this: Guild would build the 2-piece back with the strip of inlay (like mine) if the two pieces of ash were sufficiently wide. If the ash wasn't quite wide enough, they would add a strip of rosewood so as not to waste the ash.
Can't remember where I read that, but it makes sense. Also, it would explain the varying widths of the rosewood that you see. Either way, they are dymamite guitars. It was a lucky day when I stumbled across mine, and I'm glad I grabbed it.