If anyone is interested, here is the info I received from Leonard at Technical Sports One in California:

The short answer, it's Japan and they do what they do.
The cylinders are sold out and no longer available. They ran out of B and C cylinders first and unified those two part numbers to the A cylinder. The A cylinder is now sold out too.
For your B cylinder you would need the unmarked piston that they refer to as a C piston part number 13103-KV3-830.