Just ignore the numbers dude. The M68 (CCO, or close combat optic) is the US Army's designation for the Aimpoint M2.
The Aimpoint M2 and the Aimpoint M3 are externally identical, with the circuitry on the inside being the only difference.
The LT-150 is the standard Aimpoint mount designed to cowitness with the Iron Sights on AR flat-top carbines and rifles.
The LT-129 is a cantilever design, that also cowitnesses and was designed to place the Aimpoint out in front of the receiver as far as possible, in the event that there was more space on the receiver needed behind the Aimpoint, for the mounting of nightvision, magnifiers, etc etc.
So in short, if you are running a magnifier or night vision, and might want to look at the LT-129 (particularly if you aren't running a rigid railed handguard), if you are NOT running either a NV device or magnifier (which I'm guessing you aren't) then the LT-150 will suit you just fine, but the LT-129 will work as well. Cantilever mounts were all the rage a couple of years ago, but seem to have waned in popularity as more and more stationary or semi-rigid railed handguards are released, which no longer necessitates the use of cantilever designs, as you can mount your optics to the handguard rather that being restricted to mounting them on the receiver of the rifle.
(I don't work for Larue, but I love his stuff and I was bored, so there you go).