These slighty thicker o-rings are very useful in builds where the cylinder inner diameter is slightly over-sized, generally around 23.9mm+, or in situations where the piston head doesn't quite give a good seal with a new standard sized o-ring.  A standard o-ring cross section is 2.4mm, these larger ones are 2.7mm.

Cross Section: 2.7mm
Outside Diameter: 24mm
Hardness:70°