Iron Man 188 McDonnell cover 4 color separation sheets

I've always thought that the things that make up a comic book other then original inked art are vastly underrated - color guides, engraving plates, proofs, etc. which is fine with me because it has kept the prices down while OA prices have grown exponentially. These are the 4 color separation sheets for the cover for Iron Man 188. These four sheets are acetate overlays which provide the final color printed image. The top layer is the black, the second layer is blue, the third layer is yellow and the bottom layer is red. I don't see many of these (either on the market or here at CAF.com) so I felt lucky to get this one recently off ebay.