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I found this eventually:
https://justasmallblog.wordpress.com/2012/07/19/how-to-resize-a-seiko-collar-and-pin-watch-band/
I had thought the collar went in one of the outsides, and the pin went through the other. I could not get the collar to go in, so kept googling.
So, I put the collar in the middle part of the links, then holding the watch strap together force the pin back in. Helped to have a bit of metal to push the pin back in as it takes some force.
explain? I struggled with mine before i gave up... Now it sits on my sofa armrest, mocking me with its extra links.