So, after coming home with the Art of Ishgard book and many other merch items, I proceeded to enter the code in, only for the Mog Station to tell me that I'd already redeemed the in-game item.

I have two friends who also got the art book - one with the same error message, the other... got Namingway (from the Lore Book).

For clarification, both myself and my friend have the Lore Book and have redeemed Namingway. So... it looks like SE printed Namingway minion codes onto the Wind-up Relm slips for the Fanfest batch.