Cartridge / Converter fillers are more or less universal these days and many people are content to use ink cartridges, which simply push in.  If you are using the converter, there are several types.  The twist-action type and the slide-action type are just like the twist piston fillers in that a piston travels up and down the converter to fill it with ink.  The pressure-bar type is like an aerometric filler and some Sheaffer converters have a button like a button filler.