O ring Location
 

I am replacing the selector rod seal on my 1973-2.0, #12 on diagram. I am having difficulty identify where the large O ring that I also want/suppose to replace that surrounds the entire selector mechanism. Could it be # 20. If it is I am not sure where it is suppose to be seated.
https://cimg8.ibsrv.net/gimg/rennlis...a3e659dcfc.png

I believe the large o-ring you mention is #20 in the diagram. This fits at the base of the console (#10) and is used instead of a gasket. Never use a gasket to seal that console or the car won't shift properly (gaskets are used in other versions of the 901 trans, but not the side-shift 914 version).