The sliding plate circle-tow hook

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:

• The latch is made from sheet metal and fitted with a tension spring holding it forward. Modern variants generally use a wire latch with an integral scissor spring that opens the latch. Evidently the latch comes forward in front of the ring when the hook opens.
• The tow ring is placed in the latch rather than in front of it.
• There is no provision for attaching an auxilliary ring at the end of the flag, so it can't show when the latch is open. It would be interesting to know who added this feature.

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.