If I recall correctly the ones with the tail are for mag tube extensions and the ones that are hollow like a cup are for regular mag tube lengths and shortys, like the 8.5” barreled clones. The one with the tail helps to eliminate any tilting and binding where the mag tube and extension meet and the one with a hollow end will give you the maximum amount of room in the mag tube for shells.
The S&J followers are great, get the combo with the spring upgrade if you’re thinking of ordering one.