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The plate has a match to the shaft form "castellation" that you push on until it gets to a groove in the shaft, that groove allows you to now rotate the plate about 20degree so it won't now float along the shaft. The bolts now hold the plate in that orientation so that both it and the sprocket are prevented from sliding inward or outward. It's very simple but effective and easily seen if you have all the components ready to assemble, but a bit puzzling if the plate is missing.
Yes , remove cover by your left foot when sitting on bike and it's right there.Don't know about other's, but I do the bolts dry with a spot of blue lock tight to secure. They're not concentric with shaft rotation so not likely to fidget out, but because they're hidden in use then better to make sure they stay there.