Goelst sew-in adjustable curtain hooks 55mm for pleated curtains.
NOTE sold in quantities of 1, photo for illustration.
See also the Adjustable Sew-in Curtain Hooks comparison page.
Goelst used to source their sew-in adjustable pleat hooks from Microflex. As of 2016 it is unclear which manufacturer’s products they will be stocking. I do have some stock left, but probably won’t be able to get anymore :O(
If you are just looking for the hooks for Microflex pleat hooks then these will be suitable.