This item recently appeared on Kjell Scharning's site as a "new" issue, as it appeared in Michel Rundschau 4/2001. I remember something similar happening in the 1980s with two sets issued by Ivory Coast. These were listed very late by Stanley Gibbons, and images were usually of postally used examples. I'm still missing most of these sets.
Collecting this new set from Mauritania will probably be just as difficult. We need to see good colour images of these stamps, as I suspect there are a few species which are not easy to see on the image above, due to the postmarks. I think the gulls on the 10um stamp are actually Slender-billed, Larus genei. There appear to be other birds swimming with the pelicans behind the spoonbills on the 30um. The front bird on the 50 is probably a Dunlin, Calidris alpina, but there appears to be a plover behind it and others, possibly Sanderling, Calidris alba, further back.
If anyone has any further news or views about this set, please let me know.
Update identifying species on two of the stamps.
Further update showing all four stamps.