Opus was a Harmony acoustic guitar brand. By the early 1970s as a budget guitar maker Harmony was suffering from competion from Japanese, Korean and Taiwanese imports. Mandell Kaplan became  president of Harmony and decided to upgrade the product line, as they could no longer compete in the budget guitar marketplace. The Opus brand was meant to be a move up-market and Opus guitars were hand crafted from good timber. Despite the quality of the Opus instruments, guitar buyers associated Harmony with budget instruments. Opus did not turn around the company's fortunes and Harmony ceased trading in 1976.  The Opus series included four models - all had select spruce tops, block inlaid rosewood fingerboards and Grover Rotomatic machine heads. The Opus V was a dreadnaught, Opus X was a Sovereign Jumbo style, Opus XV was a grand concert and the Opus XX a Super Jumbo SJ200 style. The Opus XX had curly maple back and sides whereas the others had mahogany back and sides.

Source: The History and Development of the American Guitar. Ken Achard. 1996. Bold Strummer.