I think that depends on where you live!. To me it was an RD250LC (Race Developed 250 Liquid Cooled) but it was marketed under various names around the world. It was basically the same as the RD350LC which had same stroke and a bigger bore, the 350 had twin disc brakes at the front. With all the tuning "bitz" the 250 was good for 120MPH with tall gearing, within the rules allowed standard exhausts and carbs.
Same with the Norton. In UK there never was a 750SS. The Norton 88 (500SS) grew to the 99 (650SS). I had a friend who had a Norton 650SS Dominator in the 1970s and he was an "officionado". The 750 version was known as a Norton Atlas (in UK). Later came the 750 Commando based bikes. In racing most racing Nortons at the end of the 60s early 70s were engines made by people like Paul Dunstall in aftermarket frames like Seely. Ken Redfern used all sorts, eventually having frames made to his own design. I found this video, I knew the maker back in the day, he ran an insurance shop so he sold me my bike insurance (no kidding). H is the guy I saw racing at Croft and owns an ex Redfern Norton (or he did 11 yrs ago when it was made).
View: https://www.youtube.com/watch?v=FMkSzl44-HE