I've owned and only recently sold the Kodak DCS Proback (used on a Hasselblad V camera and a Mamiya RZ Pro-II), and Proback 645C (used on a Contax 645).
There are 2 versions of the original DCS Proback.
Version 1 fit a digital ready Camera like a Hasselblad 555ELD, and required an expensive interface plate to operate on a RZ. It did not directly work on a 503CW. The top ISO is 100, and it had two CF card slots allowing you to shoot to one card then the other. The rear panel could be angled up for low angle viewing.
Version II (Proback Plus) allowed the back to be used on a 503CW with connection sync cords. I can't recall if it offered additional ISOs.
Both versions required a Quantum battery for power unless tethered to a computer via firewire. Both were 16 meg, 12 bit square sensors.
The 645C : Contax 645 (or 645M: Mamiya 645, 645H: Hasselblad H1) are totally portable units dedicated to their respective cameras, and cannot be used on other 645 cameras. The sensor is the same size, but the camera/back is totally portable and uses a clip on Lith battery for power. The ISO selection for this back was 100 through 400.
All these backs have a RAW file format supported by the Adobe Raw Converter in PSCS and PSCS2. The Kodak software is also available on the Kodak site. The software that came with these cameras is outdated and obsolete, so if it's missing it's irrelevant.
All the cameras indicate the total number of captures to date ... but that is also irrelevant because here are no moving parts to wear out. Either it works or it doesn't. These back are built like tanks and most likely will outlive all of us.
I have moved onto a Hasselblad/Imacon H2D and can say with confidence that all these Kodak backs were ahead of their time in every way. For approx. $5,000. or less, (or $6,000 for the 645C version), you can have image quality that beats the Canon 1DsMKII, and isn't that far from the $30,000 backs on the market today.