Upon further research, I'm not so sure if these bolts were meant to stiffen up the chassis or just a cover up the weakness of the old bolts.
A Japanese blogger went ahead and swapped his old bolts for the MY2015 bolts. One of the pics shows what seems to be a broken bolt. Looks like the flange was rusted off? I'm assuming the new bolt is at the top and the old bolt is at the bottom.
Maybe the old bolt has a collar. Like those Spoon Rigid Collars used to stiffen up the subframe. Not sure. Can anyone confirm?
Source:
http://ameblo.jp/zn6-86-fa20/entry-11827527447.html