Rosemary Purdie looked at all the relevant Stylidium specimens in the Herbarium. Based on that and her knowledge in the field she came to the conclusion that the critical diagnostic that distinguishes graminifolium is that the leaves are linear, up to 3 mm wide and the midrib is clearly visible on the lower surface. In armeria, the leaves are oblanceolate (broader in the upper half), usually 4 mm wide or more, rarely 3 mm, and the midrib is not obvious on the lower surface. See http://canberra.naturemapr.org/Community/Sightings/Details/1948241