It is 10 years since I posted this query. I have solved several other queries on here after 7+ years (for myself and others), so reviving this one is only a little stretch.
A recent post on the Art Glass Collectors group by prominent US collector Louis Edwards asked about a similar item of the same size and finish and convincingly by the same maker. The base treatment of his example should dispel the impression that they are ceramic, even when relying on photos. (His photos shown here with permission.)
The questions remain: who made them, where and when?
Trevor