I can confirm that in the 1920s Harvard had very rigorous PhD oral examinations for the "comprehensives" prior to the students embarking on their dissertations.
 
I have a copy of Lauchlin Currie's diaries from 1926-27 in which he recounts in some detail the nature of his own ordeal in 1927 in front of three or four professors (including Allyn Young). I don't have the diary to hand, being out of station, but from memory he was quizzed on various branches of theory and on some very esoteric aspects of economic history. He passed, but a few days later was dismayed that his close friend Harry Dexter White (with whom he spent many hours in revision) failed. In his diary, Currie noted how arbitrary and unjust this whole exercise was: he reckoned that White was easily the most able man in his class, yet White failed while he, Currie, passed. 
 
Obviously White must have been given a second chance because he went on to win the Wells prize for his dissertation under Taussig on the French balance of payments. For this prestigious prize, White's dissertation initially tied with Currie's on Bank Assets and Banking Theory (January 1931) in which Currie roundly condemned the Fed for bringing on the Great Depression. Currie's formal supervisor was John H Williams who later told him that Gottfried Haberler (then a visitor at Harvard) was brought in to make the final adjudication and came down firmly against Currie for his criticism of the Fed.
 
Roger Sandilands