There was a difference in form between at least some 750 covers and the 850 item, even the points on cam models. The earlier cover was more rounded in front of the pressure relief valve and had a perceptible valley there and the 850 item was squarer and flatter there (as was the 850 Mk3 which had the chain inspection cap). I once filed an 850 cover to make it look more 750-like but it's mostly hidden by the gear lever anyway.
I believe that they are all interchangeable (but Mk3 requires a different oil pump spigot and rubber due to the enlarged drilling for the anti-drain valve).