Josh, the bell mouth is not used with an air cleaner. Normally, Monoblocs with air cleaners did not have a thread on the inlet side, it was plain and the rubber slipped over the stub and was secured with a spring garter. The 276 carby had a threaded sleeve which achieved the same result.
Before everyone jumps on my statement above, I am referring to the models with battery box mounted air cleaners.
Trev.