A lot of other early transistor logic families operated with such "unusual" (for today) signal levels too. One wacky thing in IBM's transistor computers is tha

A lot of other early transistor logic families operated with such "unusual" (for today) signal levels too. One wacky thing in IBM's transistor computers is that they would alternate NPN and PNP gates. This avoided an extra transistor in each gate to shift the voltage level back. So you'd end up with one gate using +12V/0V logic levels and the next one using +6V/-6V. This technique was used in three different transistor logic families, each with their own voltages. So there were 6 different transistor logic levels, plus other miscellaneous logic levels (e.g. 48V relays). IBM's old computers were a crazy collection of different voltage levels.