[AFJuvat]

Update.

Received the 2 SEY magazines and compared them to the 'UU' marked Korean magazines.

The SEY magazines are 'hardbacks', which were the first 30 round magazines produced, whereas the Korean magazines are the later 'splitback' type.

Body thickness and weight are actually pretty close to each other - within 5 grams. USGIO magazines are blued, whereas the Korean magazines appear to be painted.

The SEY spring is actually thicker and longer than the UU springs, which since I was operating on the presumption that the Korean springs were too strong.

Breaking out the calipers, I found the issue - it was the feed lips.

Though they are profiled exactly the same as the USGI feed lips, the lips on the Korean magazines were 0.002" higher on each side, thus making the opening on the magazine 0.004" wider than the USGI magazines.

Loaded the magazines and with a small brass hammer, tapped the lips profile to match the USGI magazine. No double or triple feeds after.

What is interesting is comparing my USGI 15 round magazines to the Korean ones, the Korean magazine feed lips are also about 0.002" higher, but don't double or triple feed rounds. I suspect that is due to the 30 round spring having to be significantly stronger.