I'm no expert either, but the forewing looks good for the Mazarine Blue Cupido semiargus. Either way, I can't explain the lack of spots on the hindwing. It could be an aberration, but of which species? Maybe someone who knows Spanish butterflies well could enlighten us.
Thanks for taking a look. When I posted this I only had Haahtela et al (Butterflies of Britain and Europe) and I now also have the Collins guide. I like the Collins drawings and greater description of variations but it lacks a basic description of many butterflies (Mazarine Blue among them and Osiris only a description of upper sides!). I cannot find any small blues in either book with spots on the forewing and absent from the hindwing so I think you are right that this must be an aberration.
The Finnish book describes the Osiris female fw uns as having a line of black spots almost in a straight line (as in my photo). The photos in that book show the black spots without white borders and lines at the end of the cell (fw & hw) not clearly bordered in white. Of the Mazarine it states that the black spots (on the hindwing) are white bordered and this is seen in the photos which also show the lines at the cell that are clearly white bordered. Confusingly, Collins only has a drawing of the male Osiris with spots that are not in a straight line, white bordered and the lines at the end of the cells are also white bordered. I find it hard to trust these drawings as they are at such variance from the photos in the other book. The cell lines in the specimen I snapped are not white bordered and look like those of the Osiris.
The Osiris was the only species I could find with forewing spots in a straight line. However, if this is an aberration one wonders what criteria one can hang onto for identification!
Given that it is an abberation I suspect we'll never know what it was nor will we agree on the most likely suspect.
I have had another look at this and of all the possible suspects I would favour the green-underside blue Glaucopsyche alexis because (1) the base of the wing is slightly green-bluer than the rest and Collins shows the blue-green flush as quite subtle except in the variant and (2) both my books show the hind wing spots as small and neither refer to them as being part of the essential description of the species and when I look carefully on my PC (maybe not apparent on the website) there are some very faint spots visible. The abberation then would be that the line of the forewing spots appear parallel to the edge whereas the books show it converging towards the edge.