I bought a PT25 2 days ago and it's nickel. The right side of the slide has what appears to be a QR code laser etched next the ser #. I'd say the code is approx. 3/32" X 3/32" square. It looks like a smudge to my old eyes, but under a magnifying glass it's a perfect QR code. I'm guessing it represents the ser #. I've never seen anything like this before.