So somewhere in his brain he must have had "missing rounded-top portraits of 16th century VIPs, including one of Henry VIII posed kind of like this" filed away.
This article is much better than OPs. It clearly explains what the painting is missing from (a set of busts hung in a hall auctioned separately in 1781), and even shows a drawing of what that set is.
https://www.smithsonianmag.com/smart-news/art-historian-disc...
So somewhere in his brain he must have had "missing rounded-top portraits of 16th century VIPs, including one of Henry VIII posed kind of like this" filed away.