I need help determining a 3Gs bootrom

Hello,

I can't figure what what Bootrom an iPhone 3Gs is running.

When in DFU mode, here's what system profiler says : iBoot-359.3

The model number starts with MC

The serial number's first 5 characters : 5K111

f0recast answers NO for Untethered?

This iPhone was Jailbroken in order to be unlocked so it has iPad BB 06.15.00

I'm trying to downgrade it to BB 05.13.04

Thanks a lot