Hi folks many great informations are presented here by some of the most well known Hasselblad professional and hobbyist users. I like to add a few missing informations and summarize the compatibility here for all of those who are interested in using digital backs on these timeless cameras:
Hasselblad digital backs that work on the Hasselblad 500 and 200 series cameras are:
1) Hasselblad CF and Imacon iXpress digital backs were built for universal use with the Hasselblad 500 series cameras and any other brands of camera such as the Contax 645, Hasselblad 645 H, Mamiya 645 AFD, Mamiya RZ67, Fuji GX680, ALPA, Cambo, Arca Swiss, and etc. via a specific adapter. These CF and iXpress digital backs require a sync cable to the leaf shutter of the lens. Thus in this case of interest, the Hasselblad 500 series cameras (500CM, 501CM, 503CX, 503CXi, 503CW, 500ELX, 553ELX, and 555ELD), the CF and Imacon iXpress ditial backs can be used with a sync cable to the leaf shutter of the lens. In the case of the Contax 645, Hasselblad H, and Mamiya 645 AFD cameras, the CF backs do not need any sync cable because of the modern electronic shutter are sync via the digital back gold pins already. The CF and iXpress digital backs will work with the 200 (except the 202FA which has no shutter ring to allow the use of leaf shutter lens), 1600, 2000, and 2003 series cameras if the lens have leaf shutter built in them such as the C, C T*, CF, CB, CFi, CFE lens and the shutter ring on the camera has to be put on the C position. The CF and iXpress digital backs can also be used with all the 201F, 202FA, 203FE, 205TCC, 205FCC, 1600F, 2000FC, 2000FC/M, 2000FCW, and 2003FCW cameras, but require a double cable release or using the flash sync feature in the digital back setting (see image below taken with the Hasselblad 2000CF/M Anniversary + Planar T* 110mm F2.0 F at F2.0 + CFV16 mark I digital back).
2) CFV digital backs such as CFV16 mark I, CFV16 mark II, CFV39 and CFV50 were designed to shoot more friendly without a need for the cable sync to the lens with the Hasselblad 500 and 200 series (must be modified by Hasselblad with the exception of the 201F which cannot be modified due to the non electronic shutter) cameras. Please note that only the 202FA, 203FE, 205TCC and 205FCC can be modified for CFV digital backs. Again, as stated above all of the 200 and other focal plane shutter series cameras do not require any modification to be used with the CFV digital backs if you use the double cable release or utilizing the flash sync option in the setting of the CFV digital backs. A wake up cable to the digital back is required.
More specifically regarding to the CFV digital backs and their compatibility with the built in motorized cameras (500ELX, 553ELX, 555ELD):
The CFV digital backs work on the 500ELX and 553ELX without the cable via selecting the camera as ELX in the digital back. The 555ELD will not work without the cable sync to the lens if you choose the camera as ELD in the digital back. However, the 555ELD has an add on receiver in the front end below the lens so that you can use the IR trigger remote. Once you put the receiver on and select digital instead of film you can select ELD in the CFV digital back and it will work without the sync cable to the leaf shutter lens. In my opinion, the main feature of the 555ELD is to shoot tether to the computer and especially for multi shots functions such as 4 shots and 16 shots.
To be more complete in the given information, the 503CXi and 503CW have a detachable motorizer which you can add to the camera. The motorizers on these cameras (CW motorizer) have the IR built in for wireless release.