The N-Filter is the circular tube at the bottom of the Pack, connected to the Cyclotron. It has a solid red line and nine seemingly-white holes in the diameter. It also has a rub-down transfer on its front. Its usage is unknown though speculation suggests the N stands for Nitrogen, which could be used to cool the Cyclotron.
However, in Ghostbusters 2, this mesh seems to have been removed, thus, the holes are a solid white:
One theory is that the mesh holes where filled in in order to create the molds for the second film http://www.gbfans.com/community/viewtopic.php?t=28595