Does anyone know when file names get assigned to SA parts?

It seems like the N column of the SA Parts page is always blank, but file names show up after the product is processed.

We have been grabbing part file name to create extra machining information coming from hidden part in the product. It works great for L! level parts but I can't seem to knab the file name of a SA part (my formula: ="FILENAME"&PARSETEXT("|",$N$1,1) just ends up blank in the work-order)