Freedom Arts G36 Mount Picatinny Rail. Real one to one scale of picatinny rail.
Uses #6-32 x 5/16 inch length flat socket cap screw with #6-32 nut
Freedom A\rts G36 Side Mount
http://www.fire-support.co.uk/product/freedom-art-side-mount-rail-for-g36c
#6-32 5/16 inch length flat socket screw
https://www.fastenal.com/products/details/24175?term=%236-32+5%2F16+flat+socket
#6-32 hex nut
https://www.fastenal.com/products/details/0149139?term=%236-32+nut+black