I finally managed to land myself the trousers to match an orphan jacket I've had for 10 years. It is a M-69 uniform that used the old reversible canvas cloth instead of the newly produced once sided cotton cloth. The shirt even has a military manufacturer's tag in the pocket, something that's often missing from these uniforms. Unfortunately it is unreadable.
My assumption is that the Spanish had a bunch of surplus cloth from the M-58 and shelter quarter production and they used it to make a small quantity of uniforms. Or perhaps they used the cloth for testing? Whatever the case, these items are pretty damned rare.
If anybody has any information about it, I'd love to know what you know!