Greetings,
Regarding the shell plate material, I do not know what Star purchased; but, I have two shell plates stamped "O1" which suggests O-1 Tool Steel.
You can buy tool steel, A-2, O-1, O-2, A-10, ect. in round bars. For a reasonable fee, the vendor will cut them into slices for you.
I doubt they were cast items. It would only add expense.
Cheers,
Dave