Bernhard needs to look into whether it was ohc or dohc heads that were banned.
The difference being an Inter had ohc, and the racebikes (manxs) had dohc.
Nortons had sidestepped the rules slightly, by offering lights and kickstarters as options on racebikes in the brochure,
to make them eligible.
There is a famous story about those Indian flathead racers.
After the race, the winners engines were stripped for measuring, to be sure they were legal capacity.
The Indian riders were instructed to take charge of the heads as they were removed, and sit on them !
The HD guys were kept totally in the dark about what secrets the heads held - and made them sooo fast.
(Not fast enough to beat the dohc manxs though, even if they were officially an Inter to racing spec)
It had been quite common all through racing history for singles and twins be allowed different capacity limits.
The Norton twin (mit peugeot v-twin) that Rem Fowler won the inaugural IoM 1907 TT twin cylinder race was
quite a lot larger capacity than the single cylinder guys were allowed - it was thought the twins were less efficient.
The advantages allowed to the twin cylinder guys dwindled in later years, back to parity eventually.
Took longer in the USA racing classes for that to happen....
But we diverge...