This hook, developed by B. Oksjema in 1965, is described in Sekrety Modeli Szybowcow Klasy F1A by S.Kubit.
It may well be the ancestral sliding plate hook - the type known as the Isaenko or Stamov hook depending on whose variant you saw first. More modern derivatives show some differences:
Note that, like its modern counterparts, the hook has provision for tension-controlled zoom rudder though the high tension stop, if there is one, isn't shown.
There is no sign of any provision for rudder adjustments, so these were probably all at the back of the model.