..... and with a central, polished concave pontil?
These have long been mistaken as from the Ysart stable, but you are right to suggest not. Sadly, to my knowledge, nobody has managed to identify them to date.
Also the observation about the thickness of the glass ruling them out as Nazeing is, in the main correct, and should probably be regarded as a rule-of-thumb, until more detailed information is published about the
heavy pieces produced by that factory. There is also another feature that you have not mentioned and that is the way the colour is laid in. The enamel would have been courser than that used by Nazeing, hence the 'spatter' effect more associated with Monart, and to some degree with Vasart. Furthermore, only one item of Nazeing has been recorded as having any aventurine inclusions in modern times - and this is not similar to Monart with their, often, liberal use of the 'gold stone' filings (aventurine).
Failing any confirmed ID, these pieces have long been given the honary status of Czech, but I don't believe that there is any actual evidence of that attribution.
Singularly unhelpful, I fear......sorry
Nigel