At what point was Dumbledore "Master of Death?"
The man didn't have half a Horcrux. He never possessed all three Hallows at the same time. Besides, the other three titles are actual, official ones whereas Master of Death (which he wasn't) is more of a colloquialism.
That series of clues should accept "Harry Potter" as a general answer.

Most porterhead circles agree Dumbledore was a master of death. By definition, it's possession of all three hallows (not at the same time, Harry didn't have all three at the same time, in fact no one has), and the acceptance of death. Dumbledore qualifies. The only questionable one was one of the Peverels. As for it being a colloquialism, I'd say no more than any of the others. I think most potterheads would agree there are only two masters of death, Harry and Dumbledore.

Grahame Hall didn't write Wind in the Willows, it was Kenneth Graham but this doesn't detract from the answer.

That got the Harry Potter anoraks rattled. I enjoyed the varied groupings.