Just had a look at the mystery box 2013, and found
this post there. Looks to me as if there was some confusion when some of the new staff took over of what to do with the additional material threads for the box, because some were not linked at all, and this one made an entry of its own in the box???
Moreover, the
2010 box entry this ultimately refers to has a link to an additional material thread, but that link is dead.
Just in case that confusion still persists: if an additional material thread is created, a link should be added to its box post, linking to that thread. (Not an entry of their own.) And it should look as in that 2010 box entry, except for that minor imperfection that working links would be ever so slightly more appreciated.
There are probably still quite a few additional-material threads in the mystery corner that have not been linked yet from the box.