There were 2 different Davis Rotaries---old and new style. The old style had a bobbin case set up similar to the Wheeler and Wilson D9. I have a PDF owners manual for this machine but have never actually seen one in real photos. The new style is like yours with the easily removable bobbin case. Not sure how rare the Rotary is but I only see a handful up for sale per year. Think there's one on ebay now and I'm having a hard time trying to ignore it.
Jon