Thanks for the quick responses. I've e-mailed him, so we'll see what comes of it.
I've not got the actual poster - just the image.
I'm not convinced it's that old, though. Other research shows that the boat (Princess Christine) was built in 1963, and I would have thought that the spelling of the river as "Conwy", rather than "Conway" placed it at least in the 70s.
Looking at old adverts on the site, Llandudno had 5 digit numbers in the 60s. (But this is, of course, Conwy.)
Still confused!