The chassis (the bottom black part) was always made out of metal. But all the white part above, on the very first PETs (the ones with the rectangular keyboard) it was made out of metal, and on all subsequent ones (with normal keyboard and green-on-black screens) it was made of this plastic material. Source: Personal experience (I'm old).
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Source? Every Commodore PET I've ever come across had a metal chassis. Commodore64s and VICs had plastic ones.
The chassis (the bottom black part) was always made out of metal. But all the white part above, on the very first PETs (the ones with the rectangular keyboard) it was made out of metal, and on all subsequent ones (with normal keyboard and green-on-black screens) it was made of this plastic material. Source: Personal experience (I'm old).