OK, I'm looking at getting the ARMS #40 for my BUIS, but I noticed that there is also a low profile ARMS #40L, what is the difference between them and is one necessarily better than the other.  Also, I plan on getting an Aimpoint CompML2.  Will I need a specific ARMS to cowitness it and I was told I need a QRP spacer with my mount to cowitness with the iron sights.  My problem is that there are so many types of spacers, how the hell do I know which one I need?  Thanks in advance for your guidance and wisdom.

disregard my question about mounting the aimpoint - i found a thread that answers that one.

btt

Best advice I can find regarding spacers and ARMS gear is to get a Larue Mount. You won't need to worry about it. Plus it's a little better design and quality control.

Nobody else???

The ARMS 22M68 mount was chosen by Aimpoint as a standard with their M2. If going off the receiver, you need the full spacer. If you are going to use just an Aimpoint which mounts best for use at a foward position, then you only need the ARMS #40A2 rear sight. If you may want to go to an optic that has magnification and extends over the top of the folding sight, then get the low profile #40L.  Both ARMS sights attach via a compact nut that doesn't become a knuckle buster, they also include a tube of lock tight so nothing can come lose. The ARMS products are the top notch in QC and are standard issue to US a allied militaries, and have been for many many years.
Good shootin, Jack