What follows is all speculation, but seems logical.

With the rush to develop new cartridges for use with nitro powder, it is not a stretch to believe that Jeffery and Eley (either alone or jointly) looked around and saw the 20/577. I'm not averse to thinking that someone said to himself "Why not take a brass 20 bore case and strengthen it, add length and more rim and see what it does with this new powder and jacketed bullets?"

The rest, as they say, is history.

SOS