I have written about how difficult it is to find the books from what I call the "1940s Style Library Binding."
I just acquired an example of The Secret of the Old Clock in this library binding.
It was only last month that I acquired #8, after a number of years in which I was unable to find any examples. I purchased #6 in March 2004, #5 in July 2005, and #3 and #11 in the same purchase in November 2005. Six years passed before I found #8 in January 2012 and #1 in February 2012.
Since this binding is so difficult to acquire, I have only been able to guess how old I think the books are. My estimate has been from the very early 1950s or possibly the late 1940s. The book I purchased last month was dated 1950, finally giving me one date.
The book I just purchased has no dates, since the library stripped all information from the book when it was discarded. I wish they would not do that. However, this book does have an interior list of titles. The last title listed is Ringmaster's Secret, which means that this book could not have been bound any earlier than the age of the book from which it was rebound. This places my new acquisition no further back than 1953.
Now I have a range of 1950 to 1953 for the years in which I know these books were bound.