X86-Secret : ES Zone
Pentium II 300 MHz ES  (Week 31, Year 1997)   
80522PX300512EC Q206 ES / A73100PDA0-037 A4

One of the first engineering sample of the Pentium II 300 MHz featuring L2 ECC functionality. Built on the Klamath (0.28 ┬Ám) core, this CPU feature a 512 KB off-chip cache with ECC enabled. This sample was built in early august 1997 while first P2 with ECC on only come in late '97.