Is it a option or are they installed on all 767's ?
All 767's. The Mach trimming system is found in the SAM's.
And I'm not surprised it doesn't fail. The system is solid state and I'm sure it's rarely used. The mach trim mode is only active with the flaps and slats up (when the A/P is not engaged and when you're not using manual trim).
Rgds.
NSEU.