This post was triggered by an avid birder who asked the above question.
In the late 19th century, an Oriental Honey Buzzard Pernis ptilorhyncus with an (until then) never seen before blackish-and-white plumage was collected in the southeast of Sumatra, Indonesia. This specimen was described by British ornithologist Arthur Hay in 1877, under Pernis ptilorhyncus, apparently hesitating to separate it from the very variable Oriental Honey Buzzard.
Subsequently, another two specimens of similar plumage were collected, and the distinct plumage of these birds led ornithologists to treat them as a new species of Honey Buzzard. In 1880, this new species was christened Pernis tweeddalii, in honour of Arthur Hay, who was also known as Lord Tweeddale.
Along the way, it was lumped back into Oriental Honey Buzzard Pernis ptilorhyncus as it breeds with the normal brownish-plumaged OHBs resident in the tropics. Therein lies the origin of the name “Tweeddale morph” of the torquatus subspecies of the OHB.