Hey West,
Thanks for looking.
I am thinking the difference in bracing was is not model specific, but year specific. The earliest evidence of the sound hole reinforcement plate is a pic of a top removed from an '84 D-15... then we have my '92 D-55, Tom's D-65S, and all the DV-72s that have been checked, so far (all are from '93-'94). Then, our DV-73s which go 'back" to the older style bracing in '95. I'd really like to hear from some of the other DV owners (52s and 62s, which bracket the entire date range and more), and some of the early DV-73s (from 1994) (which brings up another question I have... how many of the DV-7X guitars are represented on LTG? I think somewhere between 3-6 of the 73s, and perhaps 10-12 of the 72s? I'm just curious...).
As for the effects on tone... it does seem sort of counterintuitive, doesn't it? I don't know if it's my individual guitars, or the models, but my 73 is noticeably "brighter" than my 72 (and my D-55). I would describe the 72 as "warmer and richer." To me (don't know about others opinions), "warmer and richer" means more overtones, particularly in the low frequencies; "brighter" means fewer over tones (more fundamentals), and more emphasis on mids/trebles. Adding mass to a vibrating plate should suppress high frequency vibrations more than low frequencies... so IF the sound hole area is a major source of high frequencies, perhaps the plate there would emphasize low frequency overtones at the expense of trebles? (I am a biologist, not an acoustic scientist, so I am really just guessing)...
Anyway... I am not too worried about it from the science and engineering standpoint, the end result (with the DV-72) was phenomenal. Just for the record, I don't analyze when I play... I just enjoy the music!