Maybe I'm trying to be too smart for my own good. The rear tank, originally for gasoline, and now empty, has a shut off valve which I ignored because the tank is empty. The gasoline is in the front (main) tank, which the whole valve assembly threads up into. Its that tank which I want to turn on and off. The remaining connection just goes forward to the carb.
By the way, for all you guys who have helped me along the way here, I do have a short video of the day when we finally got the motor going. Do you know if it can be posted like a photo to this site?