S5L8920 bootrom revision for the iPhone 3GS sold between June 2009 and September 2009.
This bootrom is vulnerable to the 0x24000 Segment Overflow and limera1n Exploit, which together provide a "pwned for life" untethered jailbreak. This also means that you can restore to any iOS version that the 3GS has had without SHSH blobs. Compiled in between 2.0 and 2.1