I was aware of PR1.3.1. What had me perplexed is that there are multiple files of the type RX-51_2009SE_21.2011.38-1.XXX_PR_COMBINED_MR0_ARM.bin

(Where XXX is 002, 003, 004, 005, 006, 007, 008, 009, 010...)

Somebody pointed out those are different versions for specific regions, but I thought there were only 4 or 5 regions (Global, USA, UK, India, and Midle East / North Africa). I figured countries like Germany would use the Global image, but I suppose that's not the case.