Smudger, that's the design. It has the gap for an O Ring to sit in as a type of seal / gasket.
It needs to be an exact match and there's nowt on ebay I've seen.
I was hoping for a direct link from someone that has bought the same.
It's from a Brodex tank. Much as I like Brodex tanks / systems, they aren't the place to go for small fittings as I prefer to keep my arms and legs
Nearest I got was an old Amazon link but it's no longer available.
Thanks anyway
Ian