The black is self coloured, deep purple/aubergine colour with light through it. There is some light wear scratching under the dish but none that I can see inside the bowl - so it has spent its life in the 'best' cupboard. The design is wheel cut into the cover but not with the greatest care and attention, the lower 1.5cm has an acid frosted border. The top edge of the dish is ground and lightly polished.
I could accept 50's from condition and 30's from style but the quality is an enigma, attention to detail but not a high level of skill. The fit of the top on to the base is very good with virtually no wiggle and no wobble.
Dutch or German seems likely, as I found it here, but who might it be?