Of all the things I'd expect to fail on my 450b, the magazine was not one of them. At the range this evening, I took the mag out after a shot and heard something rattling inside. Upon opening it up, a small stamped piece of sheet metal with a spot-weld mark on it fell out. Looking inside the mag, it is one of the shims on either side just below the lips. I assume they keep the follower aligned or something. I emailed BM Customer Support since the gun's only 2 months old. I will finally get my turn in the ring with their customer support and will post results to the appropriate thread.
Looks like this:
In the mean time, I will transplant their spring and follower to a Stainless Steel 20-round AR mag I have and see how it does after a lip job.
Hopefully, they'll send me a new mag and I can keep the follower from the bad one as a spare.
Hoot