The middle screw holds the plastic shim in place that retains your comb setting.
If I recall correctly, you have to remove the comb to adjust from side to side, then put the comb on and set to where you want it then snug the forward and rear screw. You should then be able to move the shim up against the comb (or maybe it's down against the stock) to retain the comb angle and then snug up the screw. After that you should be able to loosen the front and back screw and remove the comb for cleaning and put it back on to exactly the same place it was before. It's been a while since I had one of these so I could be wrong on a detail or two but this is close anyway. It's pretty straight forward once you get it apart and figure everything out.