***
Thanks Kev.
I have waved my slightly magic shortwave uv philatelic lamp over it, and as a 'quality contro'l test, also over a Vasart concentric, an Ysart Bros concentric, and an upright frit flower in pot attributed to Salvador. The latter and the Ysart Bros gave a faint grey-green, whereas the Vasart and the 'sampler' weight gave the same quite strong blue. So I assume it was made post 1956 using some canes from an earlier time.
Alan