another thing you could do; if you have a hunger for a 10 round steel mag, is take good pictures of how the mag is suppose to be block per NJ law, then ask someone who doesn't live in a mag restricted state if they can do that modification on their 20 round mag or buy a 20 round mag and modify it like that to 10 rounds............just a thought
just spit balling here, but might be your mag has a alumium block in it and is riveted to the mag body?
i still say it might be easier to block that 15 round mag of yours to hold 10, have you looked at that at all?