The Series 4.5 ND filter shown in the above pictures is a Harrison filter that I purchased new. Later I found a series 4.5 BOLEX branded Neutral Density filter in a box at Photorama for a buck or so.
I have a bunch of Series 5, 4 and 4.5 'orange' #85 tungsten-daylight conversion filters but I don't shoot color. The ND filters are harder to find (but essential for B&W so you don't get into bad diffraction when outdoors).
Also note that all the above applies to only the "D" lenses. The "C-mount" lenses on the H8 REX all use a larger screw on hood (33mm I think) with a series 5 filter. There is a thin hood and a thicker hood and they screw together for the 35mm and 12mm lenses. The 5.5 Rex lens only uses the thin piece.
So, to sum it up:
Yvar = series 4
Switar = Series 4.5
Switar = Series 5
Also note that the H8-REX does not have the behind-the-lens gel filter holder of the H16-REX. Therefore all filtering needs to be done with the series filters in front of the lens. So you will want to make sure you have the lens hoods because they hold the filters in place.