Oh no, you're thinking of the line of smallish and rather undistinguished air-cooled 4-stroke singles that HD acquired when they bought out Aermachi. No, the RR250 was a state-of-the-art early '70s Grand Prix roadracer, water-cooled, which took the world championship with Walter Villa up. These were the same years when Harley's Sportster-based flat-track bikes, developed by Dick O'Brian, were beating everybody including Kenny Robert's Yamaha. I'll see if I can find a link.
IIRC, some of the Aermachi people started Cagiva . . .
(EDIT) Maybe this will work. Engine exterior photo on row 5:
https://www.google.com/search?q=Harl...HVsjDsYQsAQIHg