Interesting. So why did they change? And the memo is specific that it's a temporary policy, so how did it become permanent or is it still on the IBM books as a 'temporary' policy of allowing women to marry?
Wikipedia says[1], IBM experienced a growth spurt towards the end of the war. If they had any sort of hiring quota to reach, it would make sense to relax eligibility rules.
Additionally, the presidency of the company changed hands from Thomas Watson Sr to Thomas Watson Jr around that time, and he may have been more progressive. He drafted the "company's first equal opportunity policy letter" in 1953.
