I've been trying to find a used mark 4 with M3 knobs and noticed that some look a little different. Is there different generations of the M3? Also if in the future can you swap M3 knobs for M2 knobs?
You can swap them to M2 knobs, I forget the exact cost but I checked on it once.
DMG is right, the fixed power scopes use a very short knob (about half what the variable scopes M3 is) and then depending on which model/year etc. some turn clockwise and some counter clockwise on the variable models.
I don't think there is any advantage, aside in that if you do a lot of dialing it might be nice if all your scopes were the same so you always know that say clockwise was "up" etc.
For example I certainly would not want one M3 loopy that was CCW and one that was CW. That's like personally inviting Murphy to visit
I think it depends on the year and model number. My Vari-X 111 has a counter clockwise M3 knob. When I ordered a new M3 turret from Leupold they sent a clockwise dial. I had to call them and ask for a counter clockwise turret. Plus, my dial was in meters and I wanted yards, although I am not sure there is any difference between the two other than the printing.