This is a low friction high quality Stainless Steel cylinder by Prometheus the ridges are designed to dissipate heat and reduce friction resistance due to expanding o ring because of heat.
The concept of different cylinder sizes is too A: Achieve a more stable shot by matching cylinder size to barrel length. B: Although just putting type A in all guns will work, larger cylinders have more air and require more time and force to clear the cylinder of air, not installing a larger cylinder than you actually need can DECREASE ROF and return to rest speed of your piston, this in-turn increases ROF and reduces potential piston damage from gear speed / piston speed being out of sync. As you can see fine tuning your cylinder option can have lots of positive benefits on your gun build.
As a general rule 363mm barrels can use anything from A - D, so you have a lot of room for fine-tuning the perfect setup for you.
Any Tokyo Marui gun which takes a normal size AEG cylinder. Please choose type based on your tuning needs.