It probably fell off the shaft while rolling it around to assemble it, and before they got the cross shaft in. It's just one of those things you have to watch out for. The new gear should be fine. It does have the Philips head screw (gov gear retainer) installed doesn’t it? I don't trust the retainer. I install the cross shaft and use a piece of tape or something to hold it until I get the arm and carb on.
