Thanks Dick, always happy to answer questions (if I can) no matter what they are.
I've been told that they should have all deteriorated by now due to the chemicals used and the age of them but that is not the case. I thought it was due to the bad storage (in a barn for 27 years) but there were good and bad in each box so that doesn't seem to be the main issue. The ones at the bottom were in a worse state (in the main) but then there was liquid in lots of the boxes, didn't know if from rats or from the plaques leeching but the worst ones were probably at the bottom.
As to what they are made of, I'm still debating that. The "favourite" at the moment is Urea-formaldehyde but there are a couple of other contenders too. A couple of people thought they suffered from vinegar syndrome so cellulose acetate has been mentioned but not sure about that one. Also phenol-formaldehyde but I don't think that is the right one due to the type of stuff usually made with that. Acronitrile butadiene styrene, acrylic and other such substances may have an outside chance too but I think the first one mentioned is probably closest.
I have asked the new owners of BetG a number of times but had no response so I doubt I'll ever find out for certain.
The middle is made from catalin and it has crossed my mind to take one apart so I can see how they were put together mainly because the ink still seems to be fluid in some (well, it was when some were washed/cleaned) and I think that the catalin may have been shaped to the images inside with the ink put it too. I'll let you know if I do ever "open one up" but not something I'd feel that comfortable with.
Sorry if this seesm a bit long-winded but wanted to try to cover all bases by telling you all that I know/think.
Thanks for the question, take care and have fun.