"Dipped" was the closest to what it looks like inside and out, that came to mind. Doesn't look like any type of finish job, arsenal done or home done, that I've seen before. The inside doesn't have the glassy feel that the outside does but coverage is every nook and cranny.
I'm still looking for the examples I've come across like it, but looks and even feel was described the same as this 1. Researching info on this thing has been frustrating to say the least. Few examples to be found and then their all over the board.
I've probably got more pics and if not, I'm about to take it down again.