I too feel it may be a piece contemporary with Jan Cerny. He seems to have either carved away glass to give a sort of "cameo" effect or else carve into the back of glass (similar to my slab) in an "Intaglio effect. A couple of the pieces show on Google are remarkable similar to mine especially ones with two faces towards each other looking like they are about to kiss. What worries me is that he seems to have signed everything - and despite exmining and re-examining mine I can find no sign of a maker's marking. He was a significant figure in the 60s and is listed in Grover's - Contemporary Art Glass book.
All in all I suspect that it may have been made by a student of his perhaps as a form of examination piece.
Ross