iPhone Downgrading 6.0 to 5.1.1 without previous SHSH blobs??

[Applejuiced]

Replacements are refurbished phones, so it is possible that some previous owner had the SHSH's saved.

That's the only real and logical explanation.
Or only that particular phone can be magically downgraded but no other one in the world can.

 

[ArMyBoT]

'lo folks.
I've an OLD BOOTROM 3GS with 6.1.2 and I want to downgrade to 5.1.1.

I've NO shsh saved locally or on Cydia.

I read that the downgrade is even possible due to the OLD BOOTROM version of my iPhone 3GS.

The procedure suggested is to put in dfu and to restore via iTunes.
Simple but it's not working.

Any hints?

Thanks,
A.