The .577 I sold you is an early (note the two “C”'s standing for Cordite) Nitro Express & it could have had any number of bullets loaded into it by any number of makers, or retailers who wanted it loaded to their specs, and packaged into their boxes. This one was loaded for Westley’s with their L.T. designed capped bullet. (A better than average bullet variation & It think others will support me in this evaluation)
In Britain these were basically only two (large) ammunition makers Eley & Kynoch. So if perhaps Holland & Holland, or John Wilkes or another gun maker wanted .577 3" NE made, they bought it from one or the other companies. And depending on how much they wanted to pay, it could have their headstamp, packaging and loading’s, or any combination there-of or perhaps just off the shelf & they could stick an over label on it. (see below) or sell as is. As far as I’m aware Westley Richards was the only gun maker to actually manufacture their own ‘modern’ ammunition. However that was very early in the BP era just out of the paper cartridge / combustible era and only for a short time. As to who actually designed the .577? It is known with Boxer cases in several case lengths, and of course there is the Snider, which started it.