Unknown underlever pistol

Unknown underlever pistol 

With thanks to John G.

He says:

This pistol is mentioned in the First and Second Editions of the Encyclopedia of Spring Air Pistols, but is something of a mystery as for more than 15 years the maker has remained unknown, and this has been the only available picture.

Then recently another example (or possibly the same one) surfaced on Facebook, with more pictures. These show clearly the cocking process, the loading bolt, and the name “UNKAS”.

A few questions still remain unanswered - Who actually made it and in which country? Was it a prototype or are other examples known? What is going on in the dead space in the cylinder between its rear end and the barrel breech, and what are the two pins for?

The pistol has some similarities to the Hill pistol, but in that case the loading bay is fully to the rear of the pistol, so maximising piston stroke: