Huvius !
I have found an interesting article in the June 1996 issue of "Magnum" that should answer Your questions:

"The Farquharson was Selous`s favourite rifle; Gibbs made several for him in .461 calibre (Holland & Holland also built him two, in .303 and .400/.375). Selous referred to his .461 in his writings as a .450, but while many Gibbs rifles were stamped .450, they were all .461 caliber."
"A weakness of the Farquharson was the system whereby the butt-stock was attached: the receiver had elongated steel tangs top and bottom which were screwed into the wood of the pistol grip. This was prone to breaking (most hunting in those days was done on horseback and falls were common). When Selous`s cracked, he sent it back to the maker and asked for steel side-plates to be fitted to the pistol grip area. Thus repaired, it turned out to be much stronger than the original, and never broke again despite falls and rough useage. Thereafter, when ordering new rifles, he ordered them with steel side-plates already fitted. Gibbs refused to do this, but agreed to send the newly made rifles to a gunsmith named Beasley who fitted side plates. They were exquisitely done, and the contours perfectly matched the graceful lines of the stock - a work of art. I have often wondered how comfortable they were to shoot after being exposed to the African sun for an hour or two !
The idea cought on, and other customers ordered rifles with side-plates, but these Beasley made out of copper. He agreed to reserve steel plates for only three customers: Selous, "Karamojo" Bell and Major PJPretorius (for royalty and nobility he used precious metals)."

Kalunga