I had heard some story of her too, a critical, unpleasant story, but what it was I had forgotten long ago.
This is a sentence from The Great Gatsby by F.Scott Fitzgerald. The clause after the but kind of confuses me--it looks not that familiar to me, the way it's written. It's the very first time to see structure like that. Is it for some literary effect? Like emphasis? It would be more familiar to me, if it were written as . . .
but I had forgotten what it was long ago
but it was what I had forgotten long ago
What might have been the reason for F.Scott Fitzgerald to write that way?