CIMRM 828 - Castor-ware beaker. St Albans, Britain
First two images © St. Albans Museums Service, to whom many thanks. Other images taken by Roger Pearse at the Verulamium Museum, 13th August 2013. Note Mithras' garment is starry.
From: Vivienne J. Walters, The cult of Mithras in the Roman provinces of Gaul, p.40:
"Castor ware" is an alias for Nene Valley Colour-Coated ware, according to Potsherd: the atlas of Roman pottery.