Attachment 167481

I seem to have a problem with one side of a 4x5 Fidelity Elite DDS
The negative (paper) mage attached shows white marks that are peculiar to one side of one DDS.
Are they the results of pin-holes, dirty slider or what?
The paper was cut from an 8x10 sheet and the other three portions are without similar marks.
Question:- how to check slider - nothing untoward is visible on either face of the slider
Question:= how to clean slider?

regards

Tony