The HV90X was the original finder for film use, magnification 2.7
The HVD90X was for the "cropped sensor" digital backs and magnification 3.1
The HVX90-II was for 60 and 80MP versions of the H4D and H5D, "full frame" digital and reverted to magnification 2.7
The II designation means that the True Focus" indicator is present in the viewfinder which is not present in the original finder.
I think if you study closely you will see there is a difference but as it is from 2.7 to 3.1 it is not "in your face" different in the magnification the frame itself is still the same as defined by the drop in screen.The "ground glass" screen differences are in the frame lines for the various sensor crops they have no effect on the magnification nor on the field shown.
In the H6X literature it states: "HV 90x-II viewfinder optimised for the film and 40.2 x 53.7 mm format. The screen is an Acute Matte type D with markings for spot metering area and AF sensitive area.Optional screens with grid and/or format markings available"
Simply, the field of view remains the same whichever finder is used the magnification of the field changes. The HVD90X is perfectly usable for film, you will see the full frame as it will appear on film (almost it is not perfectly 100% non are in this range) you do not need the extra finder you purchased to shoot film.