Just a couple of additions:

Never heard of a Parked original GI Carbine mag.  Might be wrong, though I doubt it.

30-rd mags can sometimes be problematic, even with the essential later-style mag latch designed for them.

Remember that you're dealing with mags that are 60 years old, and don't be surprised if you need to replace the springs.   That applies to NIW mags, and even more so to ones that look used.

30-rd mags have a follower that holds the bolt open after the last shot; 15-rd mags do not.  You'll see the difference if you compare them side-by-side.  The followers can be interchanged.

If you see rust on the outside, there's rust on the inside, most likely including the spring.  Bring a small flashight and try to peep inside to inspect the spring (not easy).

All things being equal, I'd rather have 2 15-rd mags that worked than a 30-rd mag that didn't.