It's not exactly hard to prove that this used to appear in newspapers all the time. A 10 second search on the LOC website shows a ton of examples of this phrase appearing in papers across America: http://chroniclingamerica.loc.gov/search/pages/results/?stat...
Professor Jensen should be ashamed of himself for failing to conduct even the most trivial searches for this information.
News articles mentioning the phrase were exactly what Jensen claimed would have appeared if the signs existed, and whose absence he claimed proved the non-existence of these signs.
Fried just took the additional initiative of going directly to the newspaper ads, which are primary sources indicating discrimination, rather than the secondary sources of newspaper articles talking about the discrimination.
Professor Jensen should be ashamed of himself for failing to conduct even the most trivial searches for this information.