It is indeed a gun made for a one-armed shooter, or one whose lead-arm use has been largely destroyed.

Churchill's -- at least when R.C. was running things -- made something of a specialty of fitting & building guns for handicapped shooters and the XXVs were admirably suited to such.

Note the deep pistol grip for x-tra leverage. If you have an older UK version of Churchill's "Game Shooting" book you will see pics of a similar gun and also some text explaining such, if memory serves.