This used to be extremely common. It was used on virtually all Raleigh bikes for half a century, along with many other European bikes using similar seat lugs.
Find any shop that's more than 30 years old. Ask them where they buy them.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.