I used this unit with my existing reel to reel tape deck. The unit featured: mic inputs, inputs from preamp, to tape recorder, from tape recorder, to amp, record cal, A meter cal, B meter cal, oscillator cal 100mv, meter test, input levels, record level, output level, mpx filter.
Here’s an interior view of the model 100. Note that the circuit boards are removable (one removed for this photo). There were 6 boards total: 1 input board (served both R and L channels), 2 Dolby record boards (1 for each channel), 2 Dolby output boards (1 per channel) and a single output board (serves both R and L channels).