Does this shape appear in other series?   Yes although slightly smaller; Later Series 163 and  'A' Series / Kerryl A111. There is also a half-ring version, Early Series 425.

A similar posy ring also appeared previously in the Early Series at Early Series 61.  424 and 61 are both rare. Apparently it was not until the post-war era that posy rings became a popular item, in the Later Series.

Diameter; 17 - 17.5 cm.

Early Series 424 (left), with Later Series 163 for comparison.

Un-numbered example, base unmarked.