It's your "reflasher" PRS-350_Updater_1.0.00.14080.exe from
http://code.google.com/p/prs-plus/downloads/list.
I also tried to use the original sony updater. First in its untouched form, then extracted the exe and modified the *.dat to force the flasher to work (dit the same thing you did i guess withm the sony updater).
A