The Thirty-Nine Steps Why 1945 to 1970ish?

I had to decide on a 'cut off' point and made the decision that I would collect up to the time that PAN changed from their familiar yellow and black logo on the spine to the new style orange, pink and black  logo.

"The Thirty-Nine Steps" showing the change of logo styles

The Thirty-Nine Steps  The Thrty Nine Steps 

PAN G218 from 1961 / 67 PAN 330106961 from 1975