Socket 603 processors can be inserted into Socket 604 designed motherboards, but Socket 604 processors cannot be inserted into Socket 603 designed motherboards due to one additional pin being present.
CPU socket | Socket 370 | Socket M | Socket 478 | Secure Socket Tunneling Protocol | tube socket | Socket P | Socket G34 | Socket FM1 | Socket AM2 | Socket 604 | Socket 479 | Socket 3 | Socket 2 | Ohio State Route 603 | Daimler-Benz DB 603 | 603 |