I have taken apart a couple exactly the same lenses. The black screw in your second picture only keeps the larger scalloped retainer from turning. If you moved the black screw to clear the scallops, then the scalloped ring should turn off with only a little force applied. However, you only need to do that to open the shutter; if the shutter is working okay, then don't open it.
You should remove the lens from the standard by taking off the outer retainer ring in the back (your first image above). If you do not have a lens wrench, you can use the points on a dial caliper, or the points on a drawing compass. Sometimes you can use a small flat blade screwdriver, and simply push the retainer to spin it off.
Get some soft rubber erasers, a brayer pad, or a pad of Sorbothane rubber to use as a grip surface. Place the complete shutter/lens unit in the palm of your hand, and then apply steady pressure with the lens pushed onto your rubber pad, while turning the assembly, and the lens should come off. It helps to have the rubber pad on a grippy surface where it will not spin. If you are trying this with large soft erasers, then you put the eraser in one palm, and the lens/shutter assembly into the other, and press together while turning.
The slot in the rear element can be bridged with a kitchen knife or similar (ruler, thick feeler gauge, etc). Using that, you should be able to remove the rear element from the shutter housing easily. Sometimes it simply comes off like the front elements by turning it against a rubber pad, but not always. There should be a small retaining part on the inside of the rear element holder; you can use a very small screwdriver to carefully spin off that part, and then you can clean the optics completely. Be sure to note the orientation of the element, since it will need to go back in the same orientation in order to focus properly.
If the front group comes off in one piece, then that means the frozen focus is causing the front two elements to stick together. The arrow you show in your second shot is simply the top of the middle element of the lens assembly. If the front element actually came off by itself, then you would see the top of the middle element, at it would appear as a slot across the top, much like the rear element. When you re-assemble all this, that slot provides a torque location to tighten the middle element housing into the shutter, since you do not want it to turn. This middle element housing threads into the shutter.
Most likely the front two elements will come off in one piece, meaning that the first and second element have stuck together. Then it can be tougher to get them apart, though the previously mentioned techniques of solvents, heat, and gentle force should eventually allow them to spin apart. Resist and urge to use pliers, vices, or metal grippers, since those will warp and damage these small optics holders (they will likely go out of round, or worse case you will crack the optics).
Infinity stop is actually handled by the outside distance scale ring (the part with the three really small retaining screws). When it comes time to set up your infinity focus, you do that without the distance scale in place, only turning the front optic. Then you place the distance scale loosely onto the front housing, and gently turn it until it bumps against the infinity stop, and last you tighten the three retaining screws. It is recommended that you check the focus at two closer distances too, which will confirm that everything is where it should be located.
As to why bother with all this effort for a triplet lens:
http://www.gordonmoat.com/automotive_03.html and
http://www.gordonmoat.com/transportation_05.html These were shot with a similar lens to that you are restoring. The results can be quite good, even on colour transparency film.
Ciao!
Gordon Moat Photography
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