The Duopar wasn't the only model to use that tabbed washer. You may be able to find a lesser Huret derailer that has the part you need. The best ones had a "B" screw on the washer so you could adjust the angle. I bought a cheap steel Huret derailer on eBay and was able to make the washer from it work on a Duopar by bending the tab 180 degrees and turning the washer the same amount. The washer has to have little nubs on it that fit into the slots in the derailer and keep it from rotating. That makes the washer hard to fabricate.

See post #25 for a picture of a tabbed washer.