The keyway is just a slot cut in the sprocket end of the cam shaft, with a matching slot cut in the bore of the cam sprocket, a small square key fits in the slot and retain sprocket to cam alignment (might be a woodruf key - one side is semi-circular). Because of the keyway, the sprocket only goes on one way.
Mark the chain and sprocket with paint. When you remove the sprocket and chain, from the driver's side head only, you can then lay the sprocket aside, separate from the chain. Upon assembly, make sure the paint marks line up and the key will ensure everything is still timed.
Here is what it should look like:
Once the pins are about halfway out, then come out pretty easy.
Search for a post by me, titled "560SEL accomplisments". About halfway through this long thread is some timing chain info.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.