Concerted rummaging in the Basement Warehouse produced some rather old acrylic sheets:
Washing with detergent and denatured alcohol cleaned off a lot of grunge, but the yellow tint says it’s been around for a while. In fact, It Came With The House™ when we bought it three decades ago.
One sheet was a status board in an automobile machine shop:
So, yeah, that might be 70-year-old acrylic.