And single digits for (semi)pro, and double digits for entry level (just like my first OM-10).
That is how Olympus originally did it with 4/3rds. Single digits for the flagship models, E-xx for the mid grade, and E-xxx for the lower end. However that is where the seemingly obvious and simple stopped being obvious and simple. Only the E-30 was released with the two digital naming and rather late in the game to boot. And of the xxx models there within that xxx there was a numbering system for regular sizes ones (3x0 and 5x0 series) and the compact models (4x0 and 6x0). So basically all of three flagships, one midlevel, and then everything else crammed into the lower tier categories (probably around a dozen or so bodies).
Interestingly, in all the years since the original E-1 and 4/3rds into m43rds, Olympus has only released a grant total of 7 flagship models over nearly 20 years. If they hadn't had whatever aversion to even numbers they had at the time, they could have just named them E-1, E-2, etc. and they would still have two more model numbers left in the original one digit naming scheme.