The 900 is a single channel Leslie with built in reverb and a 2 piece Pro-Line portable cabinet. it was introduced in 1969 and was the first of what would become known as the Pro-Line Leslies. At the time of it’s introduction it was simply known as the PRO 900.
This model was designed to be used with a combo preamp for portable organs and keyboards.
Somewhat oddly, it’s bass rotor does not have a slow speed, so while both rotors turn fast in tremolo mode and both stop in stop mode, the treble horn turns slow and the bass rotor stops when in chorale mode.