Hello Ray,
I believe this is the base of a Powell powder box, although my experince of these is that the rim is cut and not fire finished as it would be blown into a mould. The colouring and shape appear correct for Powell as well as the ringing that you mention.
I have checked for an available reference and found a line drawing in The Museum of London publication "Whitefriars Glass, James Powell & Sons of London", Evans, Ross and Werner, the 1940 Catalogue, page 309, plate 529 (centre). The Pattern Numer is M1 - "M" for mould blown.
Years ago I had a base in powder blue version, which I sold. Eventually I discovered the relationship to Powell through my interest in their Cloudy range, which was confirmed when I obtained a copy of the 1938 catalogue - then came the Exhibitions and books. My goodness a wealth of information available nowadays

Cheers, Nigel