It has an expander bolt and as far as I can tell that is original. Diameter is between 22mm and 22.2mm depending on where I measure it :D Clamp a little over 25.4 but untightened.
I do wonder about older alloy components failing, specifically handlebars. I searched the internet a lot on the topic a few years ago and found plenty of people saying they wouldn't risk it, but very few reports of it actually happening. One considered write-up I read said they would be very unlikely to fail catastrophically, more like warp and bend.
No problem being told stuff I know. There's plenty of stuff I don't know, so better to risk it!
Pretty much everything about that advert is great. I'm not sure if many would pack 'shaving tackle' or a fountain pen for a cycling holiday these days. Also interesting to read cyclists were covering their steel components with vaseline in the winter!