Your plate most certainly has a paper 'Cepelia' label glued to the back.
'Cepelia' was a central cooperative for Polish artisans from 1949 till 1990. After 1990, 'Cepelia' was transfomed to include other entities, not least an artisan union and a foundation to protect Polish folk art.
Post-1990 labels are colourful (see the two examples below), whilst pre-1990 labels were printed with black ink on dull brown paper. (Unfortunately I've only got a remnant of such a label).
These plates or plaques were/are used as wall decorations, mainly bought by tourists. The last picture shows 4 or the many hanging in my parents' house.