Netgear CM MAC = MAC-2

I had gotten a used Netgear C6300, it had no label.  (The label is a removable card, thanks Netgear.)  Comcast could not install it without label.  I could not figure out what CM MAC was. To get Internet service and work on it I had to rent, $14/mo modem.  From my modem I could only get the MAC.  I spent hours with Comcast and Netgear but nobody could help.  The MAC address was not seen by xfinity.  Told modem was bad.  My daughter warned me about this.  She had the same problem.  She got a refurbished modem that had the MAC address, not the CM MAC address.  The Tech found the CM MAC on the packaging so they could then install the modem.  I checked out her modem/router and its CM MAC was MAC-2.  So I tried MAC-2 (d8 instead of da) and the install went on without a glitch took about 15 minutes total and speeds slightly faster the xfinity modem.  

Hours over many days to find out CM MAC is MAC-2.  All  you can do is get the MAC and nobody seems to know what the CM MAC or CM certificate is.  You would think tech at both xfinity and netgear would know what CM MAC is.  That it is different then MAC.  It appears on Netgear it is MAC-2.  On my xfinity rental it is MAC-3.