Hi
I picked one similar to this up at my local retro glass dealer; I didn't buy as it was all chipped at the corners. I believed it to be Czech from that sparkle their glass has, as far as use goes I was under the impression it was a paperweight c1960s / 70s.
Really nice to get one in good condition Ivo, I think most I have seen are all bashed and chipped at the corners, yours looks perfect.
A stab at maker would be Rosice under the Sklo Umbrella - but I am probably wrong and Marcus would have a better idea I am sure.
Adam P
Doesn’t belong on the GMB but I know of a Scottish company (c1960s) who did similar cubes in Perspex, I have other shaped weights in Perspex filled with bubbles or ink by ‘Midton Crafts Ltd Scotland’ – they are as heavy as glass