The M numbers refer to the model.
The latest incarnation from Shimano XT is the M78- series. M780 is the triple version, M785 is the double, but it isn't a double and bash.
It looks like this :
A M785 front mech comes in several configurations, but is double specific. The cage is the same diameter as a 40t or 38 T outer ring, and is designed to shift between a 26 or 28T inner to a 38 or 40T outer.
The mech you linked to is a triple. It will work for your setup if you limit the mech from overshifting,using the limit screws, and find the ideal throw point, as different mechs use different amounts of cable throw to shift granny to middle than middle to outer.
Now, the 785 is the first XT double chainset, so the M785 mechs are designed specifically to work with that.
SLX has had a double and bash configuration, the M662 for a few years, and the front mechs are designed specifically to work with a 36/22 or sometimes 34/22 configuration.
If your double and bash convert is nearer that configuration, then you would be better off finding an SLX M662 front mech.