I don't think so. Here is a picture of an unrestored, original paint 49 GMC I had an opportunity to photograph in detail. On the underside in the linked picture you can clearly see it was flat black underneath.
49 bed underside What is interesting is the word "Narva" written in chalk or grease pencil and the number "167". The topside has obviously been recoated with something so we don't know about its original finish although in blowing up the couple of pictures I have of the top surface you can see traces of the Narva Green near the edge of the sides.
49 bed topside All this makes me wonder if the wood was installed in the bed and then the bed was painted. Just one of those mysteries that make this hobby so interesting.
