It's a third viewer that is more like a periscope that looks straight up or in a rotating arc. You see the image in the RH eye piece and there's a sighting bar like on a scope. There's very little info on these that I could find except on Worthpoint. Condition is pretty nice with no damage to the case.
The optics are not too bad but there is some haze. Try to find another set... The item "Rare & unusual WW2 RAF ROSS LONDON Stereo Prism COMPOCULAR BINOCULARS STEPOACH" is in sale since Saturday, June 13, 2020.This item is in the category "Collectibles\Militaria\WW II (1939-45)\Original Period Items\Great Britain\Personal, Field Gear". The seller is "gardyne-vermont" and is located in Derby, Vermont.
This item can be shipped to United States.