Ah, thanks everyone. I found confirmation on the Beranek site that this particular pattern is still in production, and indeed designed by Frantisek Vizner in 1968. The green in my piece doesn't look as dark as the photographs on their website, but for the rest it's exactly the same. If it was designed in 1968 and still in production, it could hail from any year between then and now.
And I hasten to add that indeed, it's a very pretty piece. The internal bubble structure has a beautiful way of capturing light that comes through it. I'll have to use it soon in one of my upcoming monthly rotating window-sill exhibitions. I'll probably have enough hotworked Czech glass by now to use that as a theme.
Astrid