I am in no way an expert on SpCW but I have gathered some rather nice references over the years.
I would like to recomend a booklet from Galland Books, Stug3 Collection. There is one booklet called "Blindado Alemans En El Ejercito de Franco".
The booklet is in spanish only but have a lot of nice illustrations, among the colourplates with these diamondshaped markings. Most semms to show letters like an "M" or "E".
I would recomend that you get these booklets, they are in the same format as the Osprey booklets.