I went to the Auto parts store and bought two belts. I handed the guy the specs Gabe gave me (thanks) and he brought me a belt that was the right length, but too narrow. He tried again and brought me a couple of Gates 9319. It was a winner. Other numbers that would probably work: Dayco 17320, Goodyear 17321, and metric size 13A0815.
Gabe, I've attached a picture of how the saddlebags attach. They are NOS Motobecane bags that I got on Ebay. I had to widen the top brackets abit and move the spring and hook up higher to work on the Batavus luggage rack, but they now fit perfectly and mount or dismount in seconds.
BTW, I've posted a pic of the bikes on my profile page.