
^ re pricing. Pretty sure drugs are involved too.
Heaviest backplate is pre-'89, there's two variants. One is gold hued, the other is unfinished steel. They're the same thickness, but the gold hue version has slightly higher hardness and looks nicer. Specific models, well, too many to list. The unfinished steel is however, the exact same thickness as the gold one within expected tolerances. The gold one just has a different treating process.
The next version is slightly thinner, still unfinished steel. Hate these so much because they're prone to rust - there's no anti-corrosion on them. At all. Usually found in '89-'92ish but they're all over the place.
Third version is the '93+. Same thickness as the '89-'92ish but IBM apparently put the anti-corrosives back in the mix, since I rarely see these with significant rust damage. Unicomp's current are the same. However, IBM relaxed tolerances and may have made some die / pressure adjustments plus tooling wear/tear.
I'd agree with fohat there's almost no chance of written records. Maybe you'd find an order for the raw materials but that's about it. Each plant seems to be different in some minor or major ways, depending, and I don't know if they made all the plates in a central location or each facility had the tooling.
I'm going back to being sick now.
