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.
The Smell of Molten Projects in the Morning 
Do you nead a license to undertake archeological digs like these? ;-)
And a hard hat!
That stash came from the Black Hole, the dirt-floor compartment under the concrete patio, which was packed solid with mmmmm stuff when we bought the place. Nearly all of it ended up in a dumpster, but I knew those sheets would come in handy some day …