B767-300 FO reported the aircraft rolled about 6 feet after brakes were released at the gate; even though they were signaled by the ground crew that chocks were installed and they were clear to release brakes.

Narrative

We had to do a block turn back due to a sheared gear pin. Shortly after being parked we received the stairs attached signal. The Captain asked me if I saw the chocks in signal. I told him I did not. He then re-verified with ground crew through hand signals that the chocks were installed. He then asked the jumpseater to open the entry door; since he was a Captain on the aircraft; and the jumpseater re-verified with ground crew that chocks were installed. The Captain then released the parking brake. Almost immediately I noticed we were rolling backwards and announced; 'We're rolling; we're rolling'; while trying to slide my seat forwards to apply brakes. Then Captain applied them first; and reset the parking brake. The whole event only lasted 1-2 seconds; but we rolled backwards about 6 feet. We then noticed the load supervisor was standing on the aircraft. I don't know when he stepped onto the aircraft; but it was likely right as the plane started to roll backwards. Once the stairs were readjusted the Captain exited the aircraft; with parking brake set and me guarding the brakes; to verify that all chocks were correctly installed. I could have help guard the brakes while releasing the parking brake; but we still would have rolled backwards significantly by the time I recognized it and applied brakes.