For an overview of Flamethrower models in various games, see Flamer.
“A Flambe 450 model flamethrower, varmiter variation. Fires a short spray of extremely hot, flamable liquid. Requires specialized fuel to work properly. This model has been modified to fire a hotter mixture which causes greater combustibility.”— Fallout 2 In-game description
The improved flamer has a 5 damage upgrade over its cousin which, when combined with the improved flamethrower fuel, can do a lot more damage in a single shot, which is often required as most enemies in the flamer's range are very likely to carry burst fire weaponry lethal enough to kill the flamethrower's wielder.
Regardless what the description says, this weapon does not require flamethrower fuel Mk II to work. They were originally intended exclusively for each other, but this is not the case in the released game.
This gun cannot be found or purchased, it is only obtainable by having Algernon upgrade a standard flamer.