@Rod thanks for linking the Triangle Cancels article. At Great American Stamp Show I picked up this cover showing the "Sword of Light" definitive with triangle cancel ANU for An Uaimh (Navan, Co. Meath) paying the printed matter rate to the US, then forwarded to a different seminary location with a Fourth Bureau 1 1/2c definitive paying the printed matter rate within the US. Both cancels are mute, of course, so we cannot date the cover except roughly based on the issue dates of the stamps.
For me, it's a triple win: my first Irish triangle cancel on cover; my first example of ANU; and a nice solo Fourth Bureau use. Now I have a problem, do I display it with my triangle cancel collection, or with my Fourth Bureau solo uses collection.