GailT summed up the cleat diffs well. Note: all SPD mtb cleats work with all SPD mtb pedals. I have never heard of a 55 before, must be a short-lived design. Use-wise, I have always used 51s for off-road because that was all there was when I started using them in the early 90s. I put 56s on my wife's shoes for offroad use as she's a timid rider and wants an easy release. I also put 56s on my daughter's shoes but now she's 14 and races XC so she needs a firmer connection and I switched her to 51s to avoid unintended releases when sprinting or over rougher terrain.