Here is a list of the PS3 slim models and the base firmware it comes with (like NTA mentioned). .
120GB SLIM CECH2004A 2.70 (Base FW)
120GB SLIM CECH2001A 2.70 (Base FW)
250GB SLIM CECH2004B 2.70 (Base FW)
320GB SLIM CECH2504B 3.40 (Base FW) CECH2504 PS3s that come with the data code 1B (and higher) cannot be downgraded
here (http://www.ps4news.com/ps3-hacks-jai...k-downgrading/) is why.