Actually, the firmware is 5.1.1 and the modem firmware (baseband) is 5.16.xx
You can install the iPad baseband to unlock, but read all the warnings and consequences carefully as it cannot be undone.
Your reference to 35th week is for the year 2011 (xx135xx) but actually any phone made after the 27th week of 2011 may have the newer bb chip which cannot be flashed with the iPad baseband.