You all bring some very valid, and familiar points to this discussion. My Dad was a WWII Vet, so I have heard some of this before. (re: plastic carp)
Another thing to consider, perhaps? WE was contracted to produce x-gazillion telephones, and other devices, for the duration of the war. They were a highly efficient company, which always got things done.
Would it not be possible, or at least worth consideration, to imagine that they might, very well, have had extra metal shells, etc left over, from these contract runs, when WWII ended? It did end rather suddenly, with little announcement.
Pretty much all the other manufacturers had lots left over. That's why I can buy NOS Linkert carburetor kits, in Cosmoline, for my 45 Flathead. NOS tubes for some of my radios. Etc, ad nauseum.
Being WE, would they have not utilized parts already produced, being ever practical (and frugal)?
I've never seen a WE 'Wartime' / Signal Corps 302 phone, in person. That being said, I don't know if the Guv' mint spec'd thermoplastic, or metal... anybody got a thermoplastic one?
As I understand it, though, the reason for the Home Front shortages of previously commonly available materials in civilian production of goods, all the Scrap Drives, etc. was so the War Effort got the Good Stuff to beat The Axis powers.
In my mind, that would mean they got metal 302s, though I certainly stand by to be re-educated / enlightened on the subject. It's always good to make new wrinkles in the brain, and I am still a most rabid WWII scholar for 50-ish years.
Staying tuned to see what happens on this subject!
Best regards!