I sort of assume you have an answer but, to the best of my knowledge, the diffenrece between the two was the XM radio socket on the mount. the one 450 i saw looked just like my 550.
As a corollary, I know for fact that the mounts for the 660 and 665 are same, except for the XM connection on the hard wire. I have put a 660 in a 665 mount and vice versa. Funny thing, put a 660 in a 665 mouth with the XM antenna connected and it tells you that it cannot use the XM antenna. So, its smart enough to know its there and tells you .