I do think the Mach Trimmer works from 0.615M onwards, because you can dispatch with the thing U/S with this speed limitation imposed onto it.
And I remember that number from doing the Tech Course.
So it makes interesting reading.
Having flown the 73 300 & 700 (-700 having a redesigned wing) typical cruise mach on the -300 is 0.74 but flies happily along at 0.79 albeit not at econ cruise alt.
the -700 wing is made for 38000/39000' at M0.78 with Mmo0.82