These are United States military issue identification disks that were popularly known as Dog Tags. On the disks were the name of the soldier, serial number, blood type and religious affiliation.
The purpose of name and serial number is obvious. Blood type was put on the disk just in case a wounded soldier was unable to communicate and needed a blood transfusion. And religious affiliation was included in case the soldier was dying and had to be administered the last rights of his or her particular church/synagogue/temple.
Under combat conditions it might not be possible to evacuate a dead soldier's remains on a timely basis. In that case another soldier would take one of the two disks with him as a record of the soldier's demise and mark on a map the location of the body for future retrieval.
The second disk was left with the body to identify it at a later date when decomposition might be advanced thus making identification difficult.
Also attached to the chain is a P38 can opener used to open C-Ration cans. The small P38 was easy to lose and soldiers often attached it to their dog tag chain in order to keep one handy at all times. In the field green duct tape was used to wrap the dog tags thus preventing them from making noise and alerting the enemy.