that is the backing plate for Shimano DT shifters. DO NOT cut it they are kind of rare. a little wd40 and let it sit overnight and carefully work it off. use a small screwdriver.
you only got one with the frame? btw that is a great color
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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