Had something similar in the basement of a property we bought a few years ago. The couple who had lived there had a small time business making signs and faux stained glass windows out of plexiglass. That one could be angled or laid flat as well as standing vertical. FWIW...?

“My horn is full and my pouch is stocked with ball and patch. There is a new, sharp flint in my lock and my rifle and I are ready. It is sighted true and my eyes can still aim.”
Kaywoodie