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?
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.
[1]http://en.wikipedia.org/wiki/History_of_IBM#1946.E2.80.93196...
Almost certainly there are many newer policies in IBM which state now that marital status and gender aren't to be taken into account.
More broadly: the feminist movement.