After the initial 2001 model, Commodore used a mix of materials, with some models made of all metal and some of a metal/plastic hybrid (metal base, plastic top), according to this website: https://www.zimmers.net/cbmpics/cbm/PETx/petfaq.html (look for "WHAT MODELS OF THE PET ARE THERE?")
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).