Jeff,
It sure is weird that those two little ears were worn off, but I think you have the right idea on a fix. Installing shims or washers would do the trick, but it would be a static setup. How about seeing if there were Belleville washers in the correct size.
If they were stout enough, you could actually load the gear up, and the Belleville washers would provide tension so your adjusting screw would function in both directions and you could fine tune your backlash....just an idea.
Stuart
