Without checking ... my recollection is that, unlike today, the classes were logical and simple.
FIRST LETTER signifies type
The 'O' category I recall, means 'Open' Class signifying engine mods allowed.
The 'S' class was 'out of the box 'Sport' Class similar to Offshore Class III.
The 'R' class was for inboard engines.
The 'N' class may mean 'National' class (UK) that evolved into popular 'NE' and 'NF'
SECOND LETTER signifies engine cubic capacity.
(not the puzzling, committee led, self-declared 'hp' of today's offshore) ...
E - up to 50 cu inches (815 cc?)
I - up to 100 cu ins
N - up to 2 litre?
So ... 'ON' means cat or mono up to 2 litre engine mods permitted.