I was alerted to this Judy Bolton auction in a comment that was made on my last post. It confirms that I did in fact remember correctly that I once owned a copy of The Mysterious Half Cat with burgundy staircase endpapers. I had avoided mentioning that format in my Judy Bolton formats page on my website, since there was always a chance that I had remembered it wrong.
After all, I clearly recall seeing two Nancy Drew Sampson Low/Macdonald UK editions in dust jackets in an antique shop around fifteen years ago—#2 and #5 in Nappi art. This was before the internet and before I began collecting foreign editions. I didn't buy them and regretted it later. I went back for them a few months later, but they were gone. I now know that those two titles were never published in the UK with the Nappi art in dust jacket. I remember them so clearly, yet I must not be remembering right. I have no idea what I really saw that day, and it is one of those mysteries that will never be solved.
So anyway, I now know that I remembered the Judy Bolton book correctly. Here are the endpapers from the recent auction:
Here is a photo that includes the front cover of the book:
Notice that the print and illustration on the front cover are also in burgundy. The question is whether all books that look like this on the outside have the burgundy staircase endpapers.