Vintage Work Sign Danger High Voltage Keep Away Garage Industrial Shop Man Cave

Vintage Industrial Shop Work Sign "Danger Unsafe Roof Keep Off" Not sure exactly what this sign is made of - its not metal nor porcelain and it is quite heavy and thick (over a pound). Unsure of exact age, part of my late father-in-law's collection. Approximate measurements - 13.5" x 10" Shop worn condition - has that patina of being hung in a working industrial environment. Lots of dirt and marks. Two holes drilled thru presumable to hang. See all photos for condition.