Julien
I've measured a set of standard Honda CB750 inlet rubbers which came out at 35mm internal diameter at the carburettor end and 32.5mm where they fit over the inlet stub on the cylinder head. I've also got another set that look the same, but measure 39mm at both ends. They would obviously fit your carbs but are too big for the CB750 cylinder head. I suspect they may be for the CB900, but somebody on here may be able to confirm.
Eric