Hello !
Could you take measurements across and thickness of this piece ? Possibly lay it against a ruler or tape to show ? And I know this sounds redundant but you have looked all over it for any marks ?
I realize these next words of mine may sound like I'm on repeat But, to me it appears to be early 20th century Modernist or '60's Modernist. If it's early 20th century then I would say Scandinavian in origin if it's '60's then it could be anywhere, here or Europe. Sorry to sound so vague but there is not quite enough info here to go on.
If there are any marks at all try to photograph in a strong light so we can see clearly, please.
Best of Luck.