They are in Senft's book "Miniature Ringbom Engines". I included a link below for the Moriya Press website. I believe they are the publisher. If you are interested in stirling engines, I highly recommend the book. There are 2 other Ringbom engine plans included as well as a good conceptual description of their operation. That is the book I am using to build mine.
Here is a link to a build I did last winter. It was my version of the Ringbom. They turned out and run quite well.
I still plan on putting one for sale to help out our local animal shelter. Ringbom build.