All LM402 shells are aluminum now. If you want a Supra with a brass shell, look for LB402 (COB), LB417 (NOB), or LB422 (HB).
If you are handed a COB LM402, remove a lug and look inside the lug hole where the screw has stripped away the chrome. They are also much heavier. The aluminum alloy that Ludwig uses is referred to as Ludalloy.