On p. 426 I mention that the archaic disyllabic forms siet, etc. are already archaisms in Plautus limited to line-final position. But I should also note that siet does show up rarely in the classical period and beyond. For example the formula quod melius siet populo Romano Quiritibus occurs several times in the Commentarii of the Ludi Saeculares in the years 17 (CIL 6.877 in the Hymn to Moerae) and 204 (CIL 6.32329 in the Hymn to Juno) and in Hymn to Terra Mater (AE 1935:26). Since this clause is a recycled prayer formula the survival of the archaism siet is not too surprising, but a recently discovered curse tablet from Peñaflor dating to the second half of the 1st century BCE also has the form siet. See. A.U. Stylow. 2012. "Stumm wie ein Frosch ohne Zunge! Eine neue Fluchtafel aus Celti (Peñaflor, Pro. Sevilla)" ZPE 181:149-155.