Hi Frank,
I do not know much about this piece. However, it looks to me like it would have been cast lying down, with a triangular piece (maybe of wood or iron) present to create the hole. The marks on the top edge of the piece are very similar to cast marks I have seen on Czech and Scandinavian cast glass. The size is huge and not many places would have the facilities to cast such a big piece of glass. It seems to have also been polished along the front. A complex large sculpture. 60KG+ I am surprised it is not signed. It may be worth emailing Svoboda Ag in Karlov Czech Republic to see if J. Svoboda made it. Having said that, I wonder why Czech came to mind for you, was it sold to you as Czech? Interesting piece, you ought to get an attribution eventually!
Robert (bOBA)