My opinion is that this is still a Common Grass Yellow which has either lost its cell spots, or in a way aberrant in that it has no cell spots. The problem with these Yellows is that they are so common and variable, that no two specimens are exactly alike. Add on wear and tear and also aberrations, there are always curious specimens that turn up every now and then. I've seen "no-spot" Grass Yellows and have recorded a few in my 'museum' but they are more likely aberrations of more common species.

Note in the ID key, there is one other species that has the black border in space 1a and 1b sloping towards the base - E. lacteola. But on checking again, this is unlikely to be it cos of the totally absent cell spot, as well as the details on the border in spaces 2 and 3.