Thanks for the clarification, much appreciated! So it is the same mess just as with their CPU names.
Except for the ones that aren't. And it's not even chronological -- the HD 7510 with a TeraScale 2 chip was released after some southern isles cards.
And there's the Radeon HD 7790, which has a GCN 2 sea isles chip.
I became aware of this because my "R9 280x" turned out to essentially be an upclocked HD 7970, using a Tahiti/southern isles chip. GPU was fine, it's just a nuisance to sift through what's what...