Done this, and shows as:
HFC MAC - 00:MY:MA:CA
D:IE (obviously - my valid MAC)
USB MAC - 00:MY:MA:CA
D:IF (My MAC +1)
TFTIP - is set to the one DHCP force uses when sniffing
I have put in a random serial number (26 digits I believe)
Downstream manually set to 33100000
STILL no downstream lock
Oh it also HAS NOT reverted back to DE:AD