I came across a 1940 map of Hebei Province with county demarcations in the last few pages (412-13) of 河北省県政状況調査報告. A digitalized copy of the book is available on the National Diet Library's online archive.
Here's a link: http://dl.ndl.go.jp/info:ndljp/pid/1272832. I hope it's what you're looking for. Let me know if it doesn't work.