Cross-check all the sites above and any others you can find. Look over the thread at rangefinderforum of photos of TLRs- lots of Rolleiflexes with ID-

Things I've learned to look for, in no particular order:

* strap lugs
* focus knob size, details, and surrounding flats
* DRP or DBP on nameplate? DRGM or DBGM? This marks the transition from Reich to Bundes (?) in the name of the German state.
* the flash sync switch and the self-timer switch- position and style
* shutter release style
* shutter release lock
* EVS system- numbers on shutter dial? de-coupling switch on aperture dial?
* focus hood- sports finder? removable? style of magnifier mount?