45 years ago, before "Secret black boxes" did things to the engine, we could buy 105 Octane petrol at any garage which could take 11.5:1 in a race engine, but with today's unleaded stuff, 10:1 is pushing it.
I think you are mistaken. 5 star was generally 101 octane on sale to the public. Maybe it was available as a racing octane but unlikely for a garage/petrol station.
Fuel terminals generally stored 2 grades, 2 star and 5 star. 3 star and 4 star were mixed from the 2 and 5 star. We used to mix it on the tanker loading racks.
i remember the Rovers used to have a sticker on the rear window stating only to fill tank with 101 octane 5 star.