[G8HQP Dave]

A different number generally means different characteristics, and in most cases the same number means the same characteristics (don't mention the EL84 vs PL84!). Hence the short 'EB91' could not be called EB92 without creating confusion. It probably should have been called EB91A, but then someone might have said that a suffix could imply improved reliability or whatever. I can imagine that a committee somewhere spent many hours debating all this; I can certainly remember that back in my days as a computer programmer we would spend a few hours designing a new feature for the software and then an equal amount of time arguing about what it should be called.