This is the socket type that is used by Pentium II processors. It's a SEC type socket. Many say that its invention was merely a marketing act by Intel in order to overthrow its competitors - the design of this socket is patented and cannot be cloned.

This SEC slot was used originally for Intel's Pentium II CPUs. Later, they then used this slot for their Celerons and Pentium 3s before they began to use Socket 370 for both. (Note: This is in a similar fashion to their using SEC Slot 2 for the "P3" Xeons and then changing the interface to a FCGA socket for the newer ones.

It is probably also worth noting that AMDs Slot A interface is a mirror of Intel's Slot 1 (as they couldn't copy it -- see the above write-up). This interface was also then changed to a socket type interface: Socket A (Socket 462), which AMD used right through to their Athlon XP (Palamino) processors.