I do believe it’s customery for the Mayor of a city to send Congrats letter to parents of new borns.
Perhaps it could be discovered that was also the case back in 1961 Hawaii? And perhaps Mayor’s office have a record of this?
I’ve never heard that before and I’ve birthed my share, different towns and different states too.
I also don’t have a copy of any such letter in any of our family history boxes. And trust me, our families saved everything for our memory boxes.