some 'upright' bars are not designed to take reverse lever. especially steel ones. I can't reall tell if the 'shim' is meant to come off but I suspect it is part of the lever. the expanders are meant to press that outward to wedge against the inside wall of the bar.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk