Thinking to Some Purpose

I recently read Susan Stebbing's book 'Thinking To Some Purpose' - originally published in 1939. Apart from its interesting content, I found something unexpected on the following page.

Did you spot it? In the days of manual typesetting, misprints were more common; we have all seen them. However, I have not seen one like this before although, given the nature of the letter in question, it is a quite understandable slip and it is perhaps surprising that one hasn't seen it more often.