Writings of unidentified authorship should be listed as “Unidentified” and filed at the end of the author list in the Writings of Others series or sub-series (see the Joseph Brodsky Papers, GEN MSS 613).
If subject identification of untitled writings is useful or important, supply a title. Otherwise, just list them as “Untitled” (see the Glenway Wescott Papers, YCAL MSS 134).
When listing a physical item as “untitled,” bear in mind that some readers may understand this to mean that the work itself was never titled, which may or may not be the case. If confusion is anticipated, consider adding qualifying notes.