Adjustable curtain hooks for curtains with heading tape. There used to be a couple of variants, what I called lock and open adjustable curtain hooks.
Below are the two variants:
Adjustable curtain hook open
and adjustable curtain hook lock
Originally these curtain hooks were also available in three sizes:
However from 2014 onwards most suppliers in the UK only made the 75mm open versions available as a stock item. I still have a some stock of the 90mm variants and as I say the 75mm open version is widely available.
NOTE sold in quantities of 1, photo for illustration.