The Moog 921B Oscillator is a much improved over the 901-B Oscillator. It is driven by the 921A Oscillator Driver and has better performance, more features, and significantly better waveforms than its predecessor.
The 921B Oscillator is a 1 MU wide module. Four of the trimmers for scaling on this module are multi-turn. I suspect they have been replaced.
A close-up image of IC8 shows the Tempco R28 underneath the IC with heat sink grease for thermal conductivity to the bottom of the IC.
The rear serial number label format changed by 1975. This label indicates the module was assembled Dec. 16, 1975 and tested the next day. The date codes on the three ICs are all 1974.
This image shows the nicely formed waveforms of the 921B. There is no pulse width control as that is on the 921A Oscillator Driver.
By 16 KHz the triangle waveshape has degraded. The pulse waveform is non-existent although I am driving this on my bench without a 921A Oscillator Driver. Increasing the Pulse Width would bring back the pulse.
The LFO has a range of 2 Hz to just over 13 Hz with the front panel control so the range is more than +/-1 octave. With a -6 octave CV I could easily drive this LFO down to 29 mHz (1/34 Hz) and the waveforms are still very nicely shaped. I did not try to find the lowest frequency.