Why would locket in 12,GP be real horcrux?

[pforparvati]

No: HPFGUIDX 153076

Most of us think that the locket in Grimmauld Place must be the real 
horcrux. But we know that Harry has seen that locket in Grimmauld Place.
[GoF] Harry has also seen the  original locket around Merope's neck in 
DD's pensieve. If the two lockets would have been same, then surely 
Harry would have recognized it. But he has not. 
I mean as soon as he saw that locket in the Pensieve he would have 
wondered "I have seen this somewhere..."... if not immediately then 
at least after the cave incident...may be after some thinking...  

But this is not the case. I understand it's all our guesswork here. But 
still why are we supporting this theory, when Harry himself is not able 
to relate those two lockets.

PP, who knows this question is not even worth of penny, but still 
hoping that somebody might answer...