Aren't those filters 100x150mm so you'll cut off 20mm of the 120mm lens on the top or sides?
The Z-Pro size filters are 100mm wide , the X-PRO filters are 130mm wide and 170mm high .
So the X-PRO filters are 10mm wider than the lenses filter thread , though you lose some in the slots of the holder .
However, the adapter used to hold the filter holder to the intended lens has a maximum 112mm adapter .
This could cause vignetting, but I'm expecting the image circle to still be sufficient that it won't show on the film area anyway.
I mostly just use a bit of tilt for landscape shooting , so might not be an issue .
Most of what I'm intending to shoot on 10x8 that this lens will be used with won't need these filters anyway , still life , portraits etc .
The plastic Cokin filter holder does slot over the Symmar-s lens with the slots for the adapter gripping it .
However with the weight of the glass polariser filter a yellow filter and an ND grad or ND filter , it might be too heavy and fall off .
So if I do have a problem I might screw four plastic pegs to it to grip the lens barrel better , or turn a length of tube down to screw onto filter holder and slide onto the lens .
No doubt both methods will also require the use of elastic bands .