The GS slab could be a designer monogram for Gösta Sigvard. He worked for Lindshammar Glasbruk between 1968 and 1980. The two dots (umlaut) are possibly for the o (ö). I can’t explain the hat, can’t find any pictures or clear information of him. I see a lot of tableware and a few signed vases and bowls of his hand.
The slabs are very thick, maybe the other ones are also designer monograms. I'm not familiar with Lindshammar designers, but in that case they could have been used for expositions or presentations of their artwork, maybe they’r one of a kind ??
It’s only a guess hopefully it will help.
I was searching for my signed GS Lindshammar bowl and saw this old post.