Miked, you've posted the progression accurately. In WWI and WWII, draft existed, draftees were numbers and naked and, of course, male. Physicals (see musterung posts on tumblr) were naked men moving, or being moved, like cattle. If shoes were allowed, it was from disease concerns only. During Vietnam, which I just missed, it often depended upon the officer assigned to the draftees. Most posts seem to have men in white briefs. My own physicals date back to this period and, yes, I always stripped down to underwear only. I always took my underwear off for a hernia check or exercise check (heart rate) since these were formal, head to toe, exams which never deviated, and once underwear was off, it stayed off.
But this was also the end period of naked swimming at Y's and ivy league colleges. Wrestlers continued to weigh in naked until the 90's. Male bodies were bodies to be examined or weighed.
Then, just as briefs began to be replaced by boxers, gowns began to appear at medical offices, the formally structured physical was replaced by something friendly and personal. Enlistees are examined in underwear, but for the must be naked parts, are taken, one by one, away from others.