[Scythels]

 

Is that the case? Couldn't they just replace it with some "ordinary" optical glass to keep the same filter characteristic other than low pass effect?

In a normal OLPF the first bifringent material increases the optical path length (OPL) some - maybe .5mm or something.  Unfortunately bifringent materials had 'bad' polarization properties (i.e they happen to alter polarization instead of "purely" doing one thing... linked motions are no fun).  A wave plate then corrects the polarization state twice as much as necessary.  Finally a second bifringent material does the horizontal (vs vertical) separation and voila, OLPF.

 

In a cancellation one the back bifringent plate is just 180deg rotated so it cancels the first, as they are equivalent parts.  This maintains the longer-than-previous OPL but probably not perfectly.  So long as it is an integer number of waves away from the old one (+/- some margin, maybe up to 25-30%) results will be fine.