Just saw this post. Really nice set. I'd say that's factory paint on the interiors--I've seen lots of interiors just like that. They put the thinnest layer of paint they could get away with, often with a faint stamp, or none at all. My guess that any worker who spent time making sure the interiors looked nice probably got sent down the road. No one paid the slightest attention to the insides of these things. I've read that there was a paint store nearby & Ludwig bought any kind of white paint they had on hand, & probably watered it down to go ****her (did I get '***' for that?).
I wouldn't try wetsanding the shells, but there is an interesting post on here about getting the yellow out of WMP--might be worth a try on these as well. You're a handy guy, & could probably pull it of. btw, how big's your shop?...marko