Quoted from jibmums:

Looking at your photo of the serial number stamping on IPSND, it looks like where the black paint is flaked off around the edges, there is not only some red paint visible but also some blue paint. Can you look at that up closely and see for sure? If that's the case, perhaps the cab was printed blue first, then red, then black, like they were trying out color combos in some unorthodox method. Either way, very cool find.