To fit the cleats, you’ll need to make sure your cycling shoes are compatible with the two-bolt SPD cleats preferred for mountain bike shoes and urban riding, not the three-bolt cleats used on road cycling shoes.
There’s an arrow at the front of each cleat to make sure you attach them to your shoes the right way round. They’re interchangeable between left and right shoes.
The only clue as to which type of cleat you have is the SH51 cleats are black, whereas the SH56 cleats are silver-coloured, plus there’s an M embossed on the rear-facing tab of the SH56.
On the other hand, most riders find it relatively easy to get used to single-release SH51 cleats and the twisting-out action necessary to disengage. The learning curve is a lot less steep than with the road-going SPD-SL pedal system.
If you’re riding on more technical terrain, particularly off-road, having multiple release angles may be a disadvantage, because you’re much more likely to come unclipped when you don’t want to. That’s why Shimano’s off-road pedal range comes with SH51 cleats rather than SH56.