The next step is a little trickier. The rim of the central lens group projects only a small lip above the surface. This kind of thin-walled brass housing is easily deformed, so it is important to keep pliers and other dangerous tools well away from it. A good tool for getting a grip on the rim of the center lens mount is one of the Flexiclamp wrenches sold by Micro-Tools. I used a 1 3/16" size. Before you start unscrewing the center group, it is a good idea to make a mark crossing the rim to the body so that you can put it back without over-tightening. Also, you will want to note that the group housing comes loose in about one full turn. A friction tool made from a dowel and a piece of rubber will also work.
Once the center lens is out, you can lift off the face of the shutter and get easy access to the internal levers and gears for cleaning with something like Ronsonol lighter fluid. I also removed the back lens group with a lens spanner so as to not get debris on the lens. The whole thing should go back together pretty easily.
With the lens and shutter clean and reassembled, you are ready to shoot pictures, as long as you are comfortable with re-spooling 120 film onto a 620 metal reel. The Kodak engineers went to some trouble to design a camera that will not permit the use of film on a modern 120 spool. I experimented with using trimmed 120 reels in a film carrier from an old Agfa folder, but am happier just re-rolling 120 to 620. There is also a version of the camera which was built for 616 film.
The Monitor user manual advises that the film initially be advanced until the number "1" is just visible in the red window. You are then supposed to move the little lever from "wind" to "1-8" and slightly turn the advance knob which activates the double exposure prevention and moves the "1" into the center of the film frame window. From there you can just crank the knob until it stops without opening the red window for positioning each frame. I initially had some problems with frame spacing but taking off the top, cleaning the mechanism and adding a little lubricant got things straightened out.
Below are some images from the Monitor Six-20 shot at The National Museum of Nuclear Science and History in Albuquerque:
And one from the War Eagles Air Museum at Santa Teresa:
A manual for the Monitor Six-20 and Six-16 models is available on line at the Butkus site.