Laurabelle and OogieBoogie pointed out something on the HETFET forums that looks like it will be important later on.
On the last three TSD codes, unlocking them reveals whatever they reveal, but also something else. The titular hidden runes. The first rune (on the 4th code) was noticed pretty much straight away by everyone, but no-one seemed to know what to make of it. Unbeknownst to me (as I didn't look very hard), the next two codes also feature hidden runes, much harder to spot, but if you scour the ornamental frame of the Troll Market guide, you will find them.
After translating the Wink/Nuada letter, we've got an alphabet to work with to help us, and so far the letters are:
4th Code: U
5th Code: N
6th Code: L
I'll keep an updated track of these in the column on the right.