Nope the English didn't build guns with side clips, except of course...
Purdey

Boswell

MIDLAND GUN CO

Holland & Holland double rifleI can only think of 'em being used in the worthless sport of the pigeon ring. Spring loaded dizzy birds they were, and at the mercy of squaty rich old men who can no longer make the trek.
Pigeon shooting was an event at the 1900 Olympics. The gold medal was won by Leon de Lunden of Belgium.
Joe,
I have no idea when they came into use, but I will defer to Mr Mann's sage advice on the matter.
Pete