I have 2 340 motors. One is original to the car and the other is also a motor built in late 72. It has been rebuilt and upgraded. I was getting ready to install my distributor today and pulled both of them out and noticed one has the vac advance and the other has a block off plate. Would anyone happen to know why both of these parts (same part number and build date) are of different configurations? This blockoff plate appears to have been factory installed. Not sure? Has anyone come accross this before.