Not that there is any harm in having a nut either side of the extension but surely if the tre is done up tight into the extension and then screwed onto the rack then a locknut is butted up to that, then nothing can move?
If you did use one on both ends there must be the same chance that the locknut would come loose compared to the tre coming loose from the extension without a locknut?
Totally agree that you need a locknut on rack side to lock adjustment but i always thought the extension in effect creates a long tre that is tight and never moves. Then everything is adjusted on the rack side
Not saying im correct or anyone is wrong its just my take on it
I was gonna put a drop of loctite on the tre to extension for good measure then just use locknut on the rack threads as normal
Maybe somebody with knowledge of how westfield and the likes do it to clear up wether we need to fit them or not