Intel Pentium III 866
Celeron 850
Pentium II's came in the slot-1 packaging and early Pentium III's followed suit. These CPU's were fitted in a larger packaging to make room for external L2-cache that ran at 50% of the CPU clock frequency. At the time it was too expensive to integrate L2-cache inside the CPU-core itself.
As technology evolved the L2-cache eventually moved closer and was integrated in the CPU-core. This lowered access times and caused the L2-cache to run at full-speed resulting in faster performance. Another benefit was the use of smaller CPU packaging: the Pentium III became available for socket 370.
Most faster Pentium III's (Coppermine cores, speeds around 667MHz and faster) were built for the S370 socket. To facilitate those who already bought a 133MHz-capable slot 1 system, Intel also manufactured the faster models in slot 1 form. It's exactly the same chip but housed differently.
