Yes they are used for mayfly tails and hackles by continental anglers, although Bob Church has two nice patterns using them.
Price and description taken from
Fly-tying materials and fly tying tools and accessories - Lakeland Fly-Tying although they are out of stock on 90% on them.
Coq de Leon Lakeland Price £6.50
Sourced from birds in a very specific region of Northern Spain, these feathers have been linked with fly-tying since 1624. The fine distinctive markings on these spade hackle feathers is almost unique, it is a mottle rather than a barring. These feathers are very shiny, far more so than conventional hackles, adding an extra natural sparkle when used as either tail wisps or as a component in the wing.
Whiting Coq de Leon Hen Neck Lakeland Price £16.50
Whiting Coq de Leon Rooster Saddle Lakeland Price £16.50