On page 36 of Selous' Travel and Adventure in South-East Africa (I highly recommend this book), he describes shooting a Male lion square in the frontal skull with an Alexander Henry .450.
He shipped the skull to Alex Henry as well.

https://books.google.com/books/content?i...1428&edge=0

He owned the A. Henry and a Gibbs at the same time and used the Henry when ammunition for the Gibbs was out.

https://books.google.com/books/content?i...1345&edge=0

Selous often speaks of his Metford rifled Gibbs Farquharson as a "450" which most experts claim is just a generic term for any rifle in the 450 bore range.
I, however, think there may be more to story than that as his rifle was actually stamped "450 EX" on the barrel.
I believe it actually could have been a 450, perhaps made to shoot the 500/450 No.2 Musket cartridge because on page 169 of the same book, Selous writes of preparing to follow up an elephant with his Gibbs using "military" cartridges.
Gibbs did make some target Farquharsons to the "military breech loader" rules so perhaps it is the use of this cartridge which Selous is referring to.
The rifle does still exist - I sure would like to see a chamber cast or bore measurements on that one!!
Short of getting my hands on that rifle, which I dare say is not bloody likely, I do have something of a remedy in the hands of Steve Bertram which I hope to share soon.

Here are some of his thoughts on the 450s:

https://books.google.com/books/content?i...1036&edge=0

https://books.google.com/books/content?i...1274&edge=0