MJUC jr. is a variable-tube compressor.
It's a variable-mu design with lots of vibe. capable of smooth leveling but also heavy pumping effects. With its two gain stages and interstage transformer simulation it offers a very deep and lively soundstage.
The three-position timing switch not only controls the attack and release times of the unit, but also the slew rate of the transformers and the timing of the other parts of the circuitry, that are directly influencing the generated harmonics (saturation).
MJUC jr. is a mix of Mk1 and Mk2 of its big brother MJUC. It's combining very natural, dense compression characteristics of the early days of this type of compression with a more forward sounding approach and more harmonics creating of the variable-mu comps of the early 1960s.