This site is awesome, thanks everyone for your response.

I can confirm both shells on my Zeus are right hand threaded.

The non-drive side hollowtech cup I tried, fitted both left AND right shell on the Zeus frame. Drive side fitted perfectly; non-drive needed a bit more force but no damage occurred.

Shell width is 68mm

No markings on the new cups except Zeus and Made in Spain.

It can't be an Italian shell because the hollowtech cup I tried would just rattle around in there right? Correct me if I'm wrong.

No sign of cross-threading/damage.

Annoyingly, the old cups were forcibly removed and disposed of by my LBS before I had chance to stop him. Having them might have helped somewhat.