incase this helps others i just received this information back form Pittsburgh Modular:
We have always supplied all of our cases(Move, Cell & Structure lines) with 4-40 thread pattern nuts. And, therefore 4-40 x 1/4" screws. If there are different thread pattern nuts in the Cell 90's, then the nuts have been switched by the previous owner.
Here is link to the slinging nuts we use. They are made by Vector Electronics- the same manufacturer of the aluminum rail extrusions. No other sliding nut performs better in than the one designed to do so by Vector.
very grateful to Pittsburgh for this info, handy to know where to buy these.