Actually, the ad is kind of hard to parse; I read it as "M645 (pro bodies)", but it could also be "(M645 Pro) bodies". But yes, of course the originals only have separate 120/220 inserts, not strictly "backs".
The Pro body seems to fetch a bit of a premium, the Super less so. How important are the differences, other than the ability to swap backs? (They take the same glass, which generally speaking is not just good but ridiculously good.)
-NT
The Super, then Pro, then Pro Tl bodies (and the 645e) are a fair bit newer than the earlier bodies, so factor that in when evaluating future durability.
The finders for the older versions have some problems due to age (prism separation). The older metering finders use CDS sensors, and may also have their own problems due to age.
The older bodies are quite heavy.
Accessory availability may be better for the newer bodies (or maybe not).
The older bodies work with a standard cable release. The Super, Pro and Pro Tl bodies work with either an electrical release (same as the RZ67 release?) or a standard mechanical release with an adapter.
The most important change made between the earlier Super bodies and the next, Pro bodies is that some of the transport gearing was strengthened. Apparently there were some gear problems with Super bodies using the motor winder.
The Super has a single mechanical shutter speed (1/60) when the battery fails, but no self timer.
The Pro and the Pro Tl bodies have no mechanical shutter speed, but do have a self timer.
Most of the accessories for the Super and and Pro Pro Tl bodies work with all three, but there are some exceptions.
The Pro Tl is the only body that offers TTl flash metering.
I used to have two Super bodies and one Pro body. I sold the two Super bodies but continue to use the Pro body. I wouldn't mind a 645e body as a backup.