Actually the complete wording impressed on it is 'Holophane HG 493', so I suppose it could be Holmegaard.
It was an interesting factory tour actually, really great to see pressing going on - traffic light lenses were being made at the time. I itched to have a set of those! Also, there was a long table full of old moulds & correlating items which were made from them, that was quite thrilling to see. I think I'll put those on another thread later today.
Funny really, I'm really into architectural and industrial glass these days. Maybe I get worried that glass history for those items could be lost due to lack of interest? Still, with Nazeing's Glass Museum, some of the 20th Centurys' 'ordinary' and factory glass should be preserved now.