I have 3 of these guys. Two of them are in about the same condition as yours but the third one is close to complete, I think. I'm pretty sure you are right in that they are miter boxes for picture frames. I have never really looked too hard for more info on them, one of those things I never got around to. I am in Washington state visiting my son for a couple of weeks or maybe more but when I get back home to Canada I'll try to get some pictures to post. I think one of them does have some script on it but I don't recall what it says. If you don't see anything in 3 weeks or so send me a reminder. I have a good memory but it's really short.