I have the Skinner on a 39A Century Limited and love it. Since the Century has an octagonal barrel, I had to replace the front sight, but I wanted an ivory bead anyway. I don't think you have to on the 39M or Mountie.
As far as adjustment, the sight has both windage and elevation adjustment. A small set screw is loosened to allow sliding the peep/stem laterally for windage adjustment. Rotating the peep stem changes elevation and the peep can be screwed into either side of the stem allowing half-turn adjustments. While this might not offer the extremely fine adjustments needed for extreme range shooting with target grade rifles, it is definitely more than adequate for a .22 rimfire and its practical shooting distances and intended uses.
I have old eyes and don't like a scope on a lever gun...this sight was just the ticket for me.