I have seen them made of painted aluminum, bare aluminum, painted wood, natural wood, stapled phenolic, formed phenolic, stapled hardboard, stapled "cardboard"... with cloth hinges, leather hinges, two metal hinges, metal strip hinges... with flip latches, sliding latches, lift-a-dot latches, no latches... While a lot of the data cases were aluminum, I'd say that the majority were wood or some other less strategic material. These data cases truly epitomize the optimization of strategic materials that was so prevalent during World War II... If aluminum was needed elsewhere, they used wood... If wood was needed elsewhere they used phenolic... etc. They all have the same part number which just gives general dimension restrictions to adhere to, but other than that, the materials used varies drastically. These were used in a lot of L-birds, trainers, fighters (I believe I have seen them), and others... They are getting somewhat rare because they are pretty fragile and I'm sure a lot of civvy owners tossed them. Below is the majority of my collection. I have another, different aluminum one. The black case is 1958 probably out of a Birddog or something of that vintage.