These are Surgical Steel Replacement Balls which only fit threaded pieces, like barbells. Although the picture shows both internally and externally threaded balls, only one size and type is available for each gauge. In other words if you are ordering it for a 8 gauge or larger, it will have a threaded post, as those are all internally threded.
As with all of our Replacement Balls, due to different companies threading, we do not guarantee that they will fit the piece that you are ordering it for, even if it was originally purchased from us.
These are NOT for Screwball Rings though. You can find those elsewhere in the catalog