I am no expert on this, so hopefully others will chime in, but...
I believe it is the 40 FEL for your tractor.
The 45 FEL is for the X700 series (and predecessor X4x5, X5x5)
The 44 FEL I believe is bundled with a pto-driven hydraulic pump, for tractors that don't have the onboard hydraulic requirements.
There may be some cross-compatibility among these, too. But I think you should focus on the 40 FEL.