With 4 bolt maguras, the boosters cover all 4 bolts. With 2 bolt, the boosters only cover the top 2, and instead of having the top 2 bolting into the frame, you have some these 'elbows' which provide support by pushing against the insides of the frame.
What I wonder is, why bother with these? Why not have, on a 2 bolt magura setup, no elbows, but a booster that covers all 4 holes? That way you are completely delegating the pressure to the booster, almost.
Is this what the evo mounts do? :) The only problem that I can see is you'd have to take the whole thing apart to get your wheel off, unless you deflated the tire, which would be a pain, as it'd probably be hard to set up. But in conjunction with some custom clamps, maybe this wouldn't be a problem.
Anyone understand what I just wrote? Thoughts?
edit, crude mspaint diagram:
This is a front view, slave cylinder clamps directly into booster with a clamp holding it on. Top bolts attatch to clamp only, bottom bolts also go into frame.