Apparently they are Hofner specific, but not if you have the wrong sort of Hofner. I just got this mail from a very nice gentleman called Adrian at Barnes & Mullins, Thomastiks UK distibutor:
"Hi Stuart
You are indeed correct – the Thomastik Hofner strings are specific to the German produced Hofner basses and will only fit the German instruments.
The modern series/ignition etc – are made in China and have smaller diameter machine head (holes) and will require Hofner branded CT strings for this reason.
I hope this helps"
To be fair, Thomastik do point this out on their website on the 324H page, but like most people I suspect, I never got beyond the 'Hofner specific'' strapline at the top of the page...