I've just acquired a Rolleicord Model IV K3D from around 1954. Buying and operating these old camera's can be a little headache if you don't know what you're getting into. As a novice 6x6 TLR user I've quickly picked up some useful information from around the internet, that I wanted to share.
This particular model comes with a double-exposure lock. It comes in the form of small button that can be set to red (warning!) or non-red. The shutter will only operate in the red mode.
The model IV with Synchro-Compur M/X shutter can shutter at speeds up to 1/500 but you can't set the 1/500 speed unless the shutter is uncocked! Two simple useful things to know about this camera.
When testing the camera you can open the backdoor and release the shutter at the various speeds. Keeping the shutter in bulb (B) mode you can evaluate the state of the lens. I cleaned my lens with some rubbing alcohol and also cleaned the outside of the Rolleicord with some glycerine to clean it's exterior. Glycerin is a non-abrasive natural substance, that will also bring out some gloss into the old leather.