As I was browsing old newspapers, I came across this article. It's kinda Ohio specific, but with some creativity it could be national.
http://news.google.com/newspapers?id=KPNOAAAAIBAJ&sjid=_QEEAAAAIBAJ&pg=7315%2C3677045
This was before Ohio put out its state-specific MUTCD. It compares old school zone signage versus what was about to be used. Toledo put a twist on it as you can read in the article.
I think it depicts a time where signage was evolving into what you have now and shows how local officials sometimes put their own little nuances into signage.