"This 6th build 600 NE is the only one by any maker to have been proofed for 120 Grain Cordite, this special load recorded in the Jeffery ledger, thus making it effectively the unique and most powerful double rifle of the professional elephant hunting era."

I'm not sure this is correct.
I've seen one Jeffery 600 which is purported to be the very first one made and it has the 120/900 proofs on the flats but 100/900 on the barrels.
85lc is kindly adding the rifle to his inquiry to his Fiedkin contact so we can see what's what with that particular rifle.