Pioneer Cassette Changer Differences?

I'm having trouble deciphering the digits in the model numbers on Pioneer cassette changers.

As an example, let's take the CT-WM77R.

My assumptions are that 'CT' is "cassette", 'W' is "dual well" (in the case of changers, one multi cassette well and one single cassette well), 'M' is "multiple" (as in changer), and 'R' is "remotely operated" (hard wired to the receiver or with its own built-in IR window, both getting 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.

5's all have wired remote to the receiver, 6's have built-in IR remotes, but what does 7 signify? 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.

As I'm trying to shop for a used one that has a nice feature set (and matches the appearance of my SX-251R receiver and eventual LaserDisc player), can anyone enlighten me?