Dear Andrew

Sorry for going off at a tangent, easy done!

It is possible that the power supply was not all that it could be. It may have 
been expedient to either drop the frequency or, more probable, the draw on the 
power stations was such that the generators lost momentum at certain times 
in the day. 

This is what happens even today when a blackout is tripped - too much energy 
being pulled from the grid and the network collapses as generators can't keep 
up with the demand. To alleviate this you can drop the frequency.

I don't know enough about the power supply in Argentina I'm afraid but  
knowing that between the early 1930s and 1990s the Argentine economy 
progressively deteriorated I don't doubt that the power supply was not all that it 
should be.

The power supply in the US and most other counties would be fine most of the 
time. However during wars, economic problems, lunch time and dinner time 
(nice combination of events) things can droop a bit.

I don't think that the frequency of hum is a sure way of getting the pitch right in 
recordings but it is fairly accurate most of the time but not a silver bullet.

This or a combination with other factors could be part of the answer. This is 
why a number of recording companies kept to weight driven recording 
equipment as it theoretically guaranteed a constant speed.