From The Apple Wiki

The SEPO (Security EPOch) tag specifies the minimum Security Epoch required for the device to run the provided image. This allows Apple to prevent devices from running previously vulnerable loaders. An example of this is when Apple increased the SEPO on iPod touch (2nd generation) MC models in addition to patching 0x24000 Segment Overflow to prevent the use of the ARM7_Go vulnerability from the 2.1.1 loaders.


SoC Security Epoch
S5L8900 0x04
S5L8920 0x01 (later 0x02, then 0x04)
S5L8922 0x02
S5L8930 0x02
S5L8940 0x11
S5L8942 0x10
S5L8945 0x10
S5L8947 0x10
S5L8950 0x10
S5L8955 0x10
S5L8960 ?
S5L8965 ?
T7000 ?
T7001 ?
S7002 ?
S8000 ?
S8001 ?
S8003 ?
T8002 ?
T8004 ?
T8006 ?
T8010 0x01
T8011 ?
T8012 ?
T8015 ?
T8020 ?
T8027 ?
T2002 ?
T8030 ?
T8301 ?
T8101 ?