Increasingly rare, these are. Works great. What the heck is a ring modulator? Well, it takes your signal and generates alternately pitched waveform signals to apply to it and those disparate sound waves get summed up in unusual ways (at least to our ears) resulting in unusual ringing sonic effects from biting, metallic harmonies to totally dissonant audio weirdness. Originally designed for keyboards, Jeff Beck was the master using this particular model on guitar. See this article that describes his 50+ year relationship with a Maestro RM-1:

This one is in good, well used condition. The RM-1 was another effect that was designed by synth master Tom Oberheim for Maestro back in those days. It did work when I found it, but It needed a recap (all Nichicons), and a calibration. For controls there are sliders for output volume, Pitch (moves within a selected range) and modulation, which seems best described as an intensity control. It increases the amount of effect in relation to your dry signal. There is an effect on-off switch and a pitch range toggle for a high and low range of frequencies. In the rarest of rare, I am also including an original MP-2 expression pedal. These were designed to do the work of either the Pitch or Modulation slider. There are different inputs for each, and they are NOT designed to be used at the same time (just makes weird noises). The sticker for the Modulation jack side fell off, so I marked it myself. The expression function requires a stereo instrument cable (does not seem to work as well with an audio cable) and I am including a new 6' cable. I feel the pitch function works best with the expression pedal - can get some unsual pitch bends and dive bomb kind of effects with it. Adds some real versatility to the RM-1 for sure.As you can see, this had been modified for some unknown purpose in the past. There is an added effect LED, which is good. Then there are those two large cutouts on the top plate with screw holes on either side. Those had already been covered when I got it, and there are no modifications or other signs of work on the board, so I wonder if they were for mounting the enclosure to something. There was also a jack-sized hole in the front (maybe for a clean signal out?), that I covered with a flat black plug. So the only extra wiring in there is for the LED to grab some power from the board. One of the slider knobs was also missing, as is obvious from my replacement knob.

This works great - I have calibrated them before and seem to have found the correct settings for all those trimmers. Very fun to experiment with. I have not found a user manual but do have the schematic if you need it.