The Mysterious Stranger's .44 magnum is nearly visually identical to the Mysterious Magnum (only difference is the in-game detail of the drawings on the handle), but deals an unstoppable 9000 damage per shot, making it the most powerful, used weapon in the game after the debug megapistol.
It has no Pip-Boy- Icon. Within the Pip-Boy interface, it is referred as ".44 magnum".
It cannot be picked up again once dropped out of the inventory, due to a game engine setting that disallows pickup by the player.
If given to a non-player character (with .44 magnum rounds) through the console, they will use this weapon. However, if used by a non-player character other than the Mysterious Stranger, it will not work properly and deal no damage.
When it is drawn from the holster, it seems to play the sound that plays when the stranger himself appears in V.A.T.S.
The Mysterious Stranger's magnum will not use .44 magnum hollow point, hand load, or special rounds, only .44 magnum.