As Mark stated I had the very same thing happen to me, just bought another one and installed it very carefully and haven't had a problem since.I tried mirrors, wires and even jacked the car up as high as I could and still didn't find the gear so trying to find it isn't worth the hassle.
I talked to a guy on line that taught maintenance classes for Ford Motor Co. in the 50's and 60's to the dealers mechanics and his offical stance was to totally remove the transmission, take it apart and retrieve the gear, that's a dealers perspective, he said personally he had the same thing hapeen to his car and he just replaced the gear.