Thanks, Flakotter. It's the strap attachment that caught my eye in particular and made me think this might be a Sea Net clone. There's a thread at
http://vintagescuba.proboards.com/thread/4634/fin on the Vintage Scuba Diving Forum where a similar mask was one of the points of discussion:
Have a look at the discussion thread and see whether the masks in the images resemble your own mask sufficiently. I note that your mask lacks a strap, which may have helped if it was embossed with a manufacturer's name and would have also presumably revealed whether the strap had one buckle for adjustment positioned at the back of the head rather than being anchored more conventionally to buckles on either side of the mask.
Hope this helps. This isn't a definitive identification of the mask, just a bit of speculation so far. Masks with just a country of origin can be very difficult to trace back to a particular manufacturer. Let us know whether you think your mask looks in any way like the ones pictured in the duscussion thread I mentioned and from which the images above derive.
Britmarine