it's a secret place for bullets so u can pass them from the airport!!
i am just kidding!
it's an option backplate for the crf .12 that has some work from the inside side that help with the performance,it doesn't do much.
" kyosho rrr wc-team orion crf "
" serpent 966-mega zx21 EM "
" mugen mbx6-picco p7r evo 2 "