Here's a link to an English language manual for same. The last page is an exploded view of the gun:
http://www.eaacorp.com/Manuals/IZH-43-Manual.PDF
It could be reverse threaded, but I doubt it. The parts on the diagram that I believe are involved in your issue are 40 (the screw), 51 and 52.
Do you already have the buttstock off so that you can see into the lockwork?