A B737-500 PARKED ON A GATE INCURRED SLIGHT DAMAGE WHEN THE ACFT ROLLED BACKWARDS DUE TO FAULTY NOSE CHOCK PLACEMENT.

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ACFT WAS PARKED; ENG SHUT DOWN; CHOCKS PLACED AND BRAKES RELEASED IN ACCORDANCE WITH SOP. AFTER THE FO AND I PERFORMED THE PARKING CHKLIST; I LEFT MY SEAT TO USE THE LAVATORY AND THEN STOOD AT THE FLT DECK DOOR TO BID PAX GOOD-BYE. WHEN APPROX HALF OF THE CABIN HAD DEPLANED; I HEARD THE FO MENTION 'GATE HOLD' AND AS I TURNED TO HIM; THE FM YELLED 'THE PLANE IS MOVING.' BEFORE I COULD GET TO MY SEAT OR THE FO COULD STOP WRITING CLRNC AND GET HIS FEET ON THE BRAKES; THE ACFT HAD STOPPED MOVING. THIS WAS 7-10 MINS AFTER BLOCK IN; IT REQUIRED ANOTHER 2-3 MINS TO DETERMINE WHETHER ACFT HAD MOVED OR THE JETWAY. WE WERE ABLE TO DETERMINE THAT ONLY ONE CHOCK HAD BEEN PLACED BEHIND ONE NOSEWHEEL ON TOP OF A BARE STEEL GND SVC COVER (18 INCHES DIAMETER). THE UPHILL SLOPE AND SLIGHT MOVEMENTS OF PAX DEPLANING AND CARGO UNLOADING ALLOWED THE NOSEWHEEL TO PUSH CHOCK ASIDE AND ALLOW ACFT TO ROLL AFT. L1 DOOR AND ESCAPE SLIDE CAUGHT JETWAY CANOPY STRUCTURE BRINGING ACFT TO SLOW STOP. NO INJURIES AND MINOR COSMETIC DAMAGE.