Sorry if I got you confused. It's the one on top of the cartridge, above the ink tank (not the sponge). It is factory sealed, a plastic little ball or something. Some people get it pushed in so there's a hole for refilling, or (in my case) a hole for flushing the cartridge.
P.s. sorry for my english, it's not my first language
P.s. sorry for my english, it's not my first language