I found a couple (like 20) neon sign transformers in the dumpster in Europe. The body of them was a mixture of sand and some resin (the actual transformer was surrounded by the resin/sand mix, giving it a sturdy cover, making it semi-waterproof (the lid was injection molded plastic with mains disconnect contacts but had no seal).
Probably again to save resin with some nice side effects (more robust shell, less shrinkage when curing)
Looks pretty much like the lower one of these: