What also struck me as rather odd, is that on the old Hohner 24 bass that is one of the instruments I bought to learn on, there is no E button but there is a Bb and an Eb -this seems rather odd -the Em is a bit of a necessity for joining in a session, the small portable nature of the instrument being ideal.

The only explanation I can think of is perhaps that the instruments developed in the brass band era when the flat keys were used a lot.