You might know this, but just for clarification:
Triaxis V1 with the 1.0 MIDI board and the Marshall-esque LD1 (0001-1630)
Triaxis V2 with the 2.0 MIDI board and the Recto-esque LD1 TX4 board without the phat mod (1631-5199)
Triaxis V2 with the 2.0 MIDI board and the Recto-esque LD1 TX4 board with the phat mod (5200-8278)
Triaxis V2 with the 2.0 MIDI board and a new TX5 Marshall-esque LD1 (8279+)
This used to be on Mesa's website, but since Gibson bought them, it's been taken down.
Phat mod is performed by removing some parts from the TX4 board and changing a few overall values. If you add them back, you get the non-phat version. Totally separate board and fairly easily removable to perform the mod. Image here on what to replace:
Example image of a stock phat mod. R46, C12, R9, and C6 missing is usually the telltale sign it's a phat mod: