nodeko curtain double hook for pleats. One finger is slid through adjacent loops in the curtain heading tape and turned in on itself to make a small pleat. Very popular on smaller installations such as boats, caravans, motor homes, trucks and planes.
NOTE sold in quantities of 1, photo for illustration.
Several other companies make similar double hooks for pleats, although they are all slightly different. The photos below show comparisons from different angles.
nodeko Curtain Double Hook comparison
There is one more variant that I know of, though it is slightly larger than those in the comparisons and that is the Regency Pleat Curtain Hook