Ok what rifle are you looking at it for? What type of gas block? There are two different gas block heights with rails that I know of. One is lower than the main rail and the other is the same height. Maybe "L" means level. I will try to look it up!
The "L" is for the taller gas blocks, the "non-L" is for a low-profile railed gas block
Do you have a M&P 15 OR or do you have a gas block that someone added to a different model M&P? If you have the OR, then you have a Rail Height gas block. That means you will want a same plane front sight, not a gas block sight that is made for a lower sitting gas block.