Both / either. The primitive design, which predates 1800, utilized a wrapped fiber gasket. The newer, more modern type uses an O ring.
Your comments regarding the difficulty in finding suitable tinder are somewhat valid. While an experienced individual will go prepared and bring good tinder with him, the fire piston has the ability to dry damp tinder. Since the heat of ignition is caused by compression, tinders such as cattail fluff can, with successive compressions, be first dried, then charred, then brought to coal formation.
In addition to charcloth, natural tinders that can be ignited in a fire piston include, punk, tinder fungus, milkweed ovum, cattail fluff, cottonwood/poplar cotton, mullein pith, etc.
For more information on the use of uncharred natural tinders please see:
https://www.muzzleloadingforum.com/threads/using-natural-tinders-alternatives-for-charcloth.114322/