Soldato
its an old style fitting.. how did you miss it when buying? new post or new saddle for you.
http://www.jejamescycles.co.uk/etc-steel-254mm-seatpost-silver-id26906.html
You need that style but in the correct diameter. It should be stamped on the old post you still have.
Not a massive range of sizes available so you may be stuck with the newer clamp style.
I'd stick with the old saddle as that is a joke.
I purchased one of these when I first started cycling as an adult. I stopped using it after a few rides as it was hurting my hips. In fact, I may still have it somewhere - if I can find it, you're welcome to it.
I'll have a look for it when I get home - I'll send you a message via trust if I do still have it.