Oddly, the HD28 was introduced the same year as the less than spectacular D-76. The M series, also with scalloped braces, was introduced in 1977, and the OM28 came a couple of years later. The M series was inspired by the retopping of archtop F guitars, and Eric Schoenberg played a decisive role in the reintroduction of the OM series.
Not sure, but I believe much of the credit for the HD28 belongs to Mike Longworth who was originally hired as an inlay expert for the reintroduced D45, in 1968. The plinky-plunky Japanese guitars of those days, Ibanez and Yamaha, posed no serious threat to Martin; I believe much more important was the competition from small shop builders, like John Gréven, who based their designs on vintage guitars.
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