Such mags are theoretically legal, yes, but according to how I read the law, the trick is that you have to somehow prove that the magazine itself (or the revolver cylinder or speedloader... on topic? ) is over 50 years old. This is kinda tricky to do with a non-serialized and often completely unmarked sheetmetal doohickey.
Hello... Legal Basics: Burden of Proof is on Prosecution, not Defense.
They need to prove that it is newer.. not the other way around.