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Interesting that the key does not look like it has a keyway in the shaft. Possibly a step turned on the shaft which would make it a larger dia than we can see but still a very flat key.Maybe there is some weight reduction going on in the web and the plate is simply to give a neater look by covering the voidBack to the oilers, the photo would suggest that they screw to the darker "plate" which sits ontop of the cap and may not be screwed directly into the cap which could be covering a trough with several oil holes down to the bearing to spread the oil across it's width. Hope you have been to specsavers so you can see to make those oilers!