During my kayaking years in Poland (1974-1989) I owned and intesively used two double folding kayaks, "Neptun", made by Niewiadow (one made in early 70's, second one about 10 years later). I don't have any specific information, manuals, etc., about these kayaks here in Colorado where I am living now but I can try to answer any questions. I understand that these kayaks were similar in construction to Kleppers.

I remember that they produced also a single kayak. Have you got single or double?

You can search archives of this forum. There were some postings by owners of Niewiadow kayaks.