Just a quick correction: only the five ongoing Superman titles used the triangles—even when other titles like Steel crossed over directly with the Superman-titles (like the aforementioned Millennium Giants) Steel would not be part of the triangle numbering. Secret Files and Annuals were also excluded from triangle numbering, although certain one-shot specials (like Superman Forever or Save the Planet) were not.
During the New Krypton era (2008-2009) brought the triangles back, and they were included on Supergirl and the Secret Files one shot.