As I tried searching the actual Eorzea Database for this item and came up with nothing (it's not even shown at all on the Lodestone), yet is listed on Gamerescape, I'm going to chalk this up as an item that SE probably did plan on introducing in 3.1 and thus included in the patch files... only to change their minds about using it at the eleventh hour, too late to change the Patch Notes, and thus they were dummied out. It's possible they might bring them in at a later date, but at the moment, I think they're just an item that is in the files and not actually used at this time.

If I had to hazard a guess about where they would eventually be acquired, besides a Fanfest reward as PotatoTree mentioned, a high-level FATE reward in Heavensward is also another possibility (like how the Behemoth Earring is received as a reward upon getting a gold medal on completion of the 'We Fought a Dzu' FATE in Coerthas Western Highlands.). Either way, SE will add it in when they feel it's necessary.