C_TransmogCollection.GetAppearanceSourceInfo returns link of base item
 

The sixth return of C_TransmogCollection.GetAppearanceSourceInfo (item link) returns the base version of the item for certain items. The appearance ID and transmog links seem to be correct.

For example, I have the mythic version (lv 710 epic warforged) of this item:
http://www.wowhead.com/item=110053
The function returns the link of the superior quality lv 630 normal version.

The same seems to go for all artifact weapons, though they are not meant to be transmogged, I guess, so I can see why they would not be supported.

Relinking transmog links seems to rely on this value.

The new 7.1 function C_TransmogCollection.GetItemInfo also does not seem to return anything for the items affected by the above issue. (does for the base item, though, but not the legendary base items)