Sorry for my ignorance but what modification was done to it?
In essence, the modifications are supposed to make the meter respond to light closer to how BW film would, specially if you're using filters.
From KEH:
ZONE VI MODIFICATION: Calibration of meter to within 1/16th stop of linearity of the entire range of light values, four corrective filters to match meter's spetral response to that of the film, infra-red absorbant coating to eliminate reflections, installation of step-down baffles to eliminate flare, superior photo-electric cell, zone sytems dials. This modification offers greater consistency and accuracy, ideal for Zone System applications.