The internal or hamerless j frame was reintroduced in 1989 and designated the 640. The first 1000 produced were the "centenial" and ONLY the first 1000 carry the CEN prefix serial number. If you have one hold on to it. I worked at a shop/range at the time and our S&W rep was able to secure a low serial number one for me. Great gun, I still carry it off duty a lot, even 20+ years later. I hope this does a little to clear up the CEN questions.