Colchester, Period VI, Pit A21 - a small hollow filled with ash and charcoal. Evidently, the helmet fragments were dumped in a sack, following the Roman clear up after the Boudiccan destrucion.
Hawkes, C.F.C. & Hull, M.R. (1947), "Camulodunum. First report on the Excavations at Colchester, 1930-39", (Reports of the Research Committee of the Society of Antiquaries of London, No.14, Oxford), p.336-7 & fig 62.