Socket-604 is a 603-contact CPU socket used by some Intel Xeon processors.
The Socket-604 has 604 contact holes, the empty contact hole is used for a dummy pin. This dummy pin allows Socket-603 and Socket-604 CPU's to be used in Socket-604 mothterboards, while preventing Socket-604 CPU's from being utilized in Socket-603 motherboards.
The Socket-604 is a Pin Grid Array (PGA) Zero Insertion Force (ZIF) socket.
A ZIF socket features a lever on one side of the socket. When this lever is pulled up, the spring contacts inside the socket are opened. This allows the CPU to be inserted easily into the socket. The lever is then pushed down, closing the spring contacts and clamping the CPU into place.
For full specifications on Socket-604, refer to mPGA604 Socket Design Guide.