No horizontal stitching on the knees and no size tag either.
Interesting. These trousers are a differant variant than what I was expecting. What you have there are a pair of South African Recce Portuguese "copy camo" trousers. They were made specifically for cross-border ops, and were designed to be sterile. As produced, they only have a small size tag, and sometimes a paper tag denoting the copy pattern. Please see the following thread. Halfway down you'll see Gilles posted photos of his Recce copy Port shirt.http://iacmc.forumotion.com/t3791-unknown-portuguese-uniform
The trousers that go with this shirt usually lack side pockets, and have heavy reenforcement at the knees.
Your trousers are a slight color variation, which I've seen only seen a couple examples of. Notice that the green is much brighter on Gilles' example. The color variance of your trousers isn't just due to fading, as I've seen new examples with that same muted green. You'll see the pattern is the same though. I can post some photos once I'm at my home computer.
This Recce copy pattern is quite distinct. It's made to replicate the Portuguese M63 lizard pattern and is a close copy. However it lacks the fine detail of the original, with the shapes appearing more thick and blotchy.
How on Earth that ended up in a pile of scrap Soviet uniforms is a mystery.