Glass bead on a white-metal slip-knot ring

Black opaque glass bead, diam. 1.1cm, short barrel, with opaque white zig-zag trail marvered in, preserved with fragment of silver ring, broken in two and incomplete, diam c. 2.3 cm. with knotted twists, 2 on one side and 3 on the other.Mayer card attributes to Barfreston grave 32, and refers to Inventorium Sepulchrale p. 140, but no mention of this in text. No beads in grave. The bead from grave 34, on a ring, was red and white. The bead on pl. VII.11 from grave 34 looks very like, however, in size though it is red and white not black, and the ring is similar. Could the bead have changed colour, eg. during blitz. No card for any missing bead from this grave.