I don't believe that customer owned equipment will show up on the website.
Your belief would be wrong (although I don't think that Comcast bothers to keep track of the serial numbers for customer owned equipment).
I own both the DCM202 and the SB6121 modems. The SB6121 is my primary modem, and I use the DCM202 as a "hot-swap" spare. -- We can never have enough of nature. We need to witness our own limits transgressed, and some life pasturing freely where we never wander.