Early G-37s (like my '74) had a one-piece mahogany neck with plain silkscreened headstock logo. This changed to multi-piece maple neck with chesterfield logo sometime in the late 70s (think I have seen a couple '76s with 'hog necks on eBay, so after that). The D-30 was introduced in 87, the year after the G-37 was discontinued. My understanding is that is had much the same specs as the late G-37s, but (according to Beesley) had scalloped bracing and the new headstock design (which is similar to the older "domed" style, but tapers to be narrower away from the neck (as opposed to the olde style which is wider away from the neck).