Just to reiterate, in the case of the M520 and M540 pedals, there is no structural difference between the 2 in spite of the price difference. The exploded parts diagrams are here
http://techdocs.shimano.com/media/te...9830608616.pdf
http://techdocs.shimano.com/media/te...9830608614.pdf
and the only difference is part #2, the lock bolt.