Pioneer SX-251R compatibility question

Is this a little too obscure a topic for this forum, or did I pick the wrong category to post in?

Rar1: Exactly so. What I don't know is whether or not I will be able to have the receiver relay the extended command set of the CU-TO11 remote (also SR marked) that came with the CT-M6R to the CT-M50R changer that doesn't have its own IR window like the CT-M6R did. Both changers were made at the same time, but I am hoping that I am not stuck with the more limited set of changer commands that the SX-251R's remote has.

One thing I'm having trouble with is deciphering the model numbers on the changers. As near as I can tell, 'CT' is "cassette", 'M' is "multiple" (as in changer), 'W' is "dual well" (in the case of changers, one multi cassette well and one single cassette well) and 'R' is "remotely operated", either by cabling to the receiver or with its own built-in IR receiver, both having the SR symbol on the front panel.
What is confusing me is what (if anything) the digits signify. They don't appear to be in chronological order, but the first (or only in some cases) digit seems to be either a 5, 6, or 7, where 5 is wired remote to the receiver, 6 is built-in IR remote, but what does 7 mean? And is there any meaning to the second digit? Like, is there any difference between the CT-M5R, CT-M50R, and CT-M55R? And so on.