If I were a betting man I would put money on the brass knocker to Dumbledore’s office being a relic also.
In fact if you were asked to describe Dumbledore’s office door, when lined up with other doors, you might describe it as the “Griffin Door.” The knocker would be the “Griffin Door Knocker.” We do know how JKR likes to play with words. As with many of JKR’s clues the Griffin door knocker was introduced early in the series, CoS, received one mention in GoF (just as a brass knocker), but received at three mentions in OotP and one mention (just as a brass knocker) in HBP.
Wow, Glumbumble, that is genius. Exactly the sort of "hiding in plain sight" that we should expect. I had been thinking of Tom Riddle's silver cup, but the brass knocker makes more sense.
The mention that the sword is the only "known" artifact of Godric Griffindor's is just too much of a tease.
It is now notable that both of the memories revealed in "Lord Voldemort's Request" begin with references to doorbells and door knockers:
"A tinkling doorbell rang and both mistress and elf jumped."
"The younger Dumbledore seemed to be waiting for something, and sure enough, moments after their arrival, there was a knock on the door and he said 'Enter.'"
...AND NOW THE TWITCH HAS MEANING!!!
"Voldemort stood up ...
For a second, Harry was on the verge of shouting a pointless warning: He was sure that Voldemort's hand had twitched toward his pocket and his wand; but then the moment had passed, Voldemort had turned away, the door was closing, and he was gone."
This scene must have taken place just at the door as Voldemort stood up to depart. It could make sensing that Voldemort was, in fact, making a horcrux of the knocker at that moment!