Aziz:
That chambering is more universally known as the ".400 Purdey", perhaps to avoid confusion with the .450/.400 x 3-inch cartridge.

A friend of mine, Vic P_, shot many buffalo with a couple of doubles so-chambered back in the meat-hunting days, though he would be the first to point out its limitations for today's DG hunting. Perfectly satisfactory for medium game, though.