Please what is the purpose of this little hidden place?

by Marga Dobma
(The Netherlands, Helmond)

And please what is the meaning of this hallmark? And of what periode this brooch is?

I bought this little blue enamel brooch with this hidden place. Is has no other hallmarks. I tested the stones as little rose cut diamonds.

What can you tell me about it?

Thanking in advance,

Marga Dobma