Jolyon S Hudson wrote:
> 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.

I think you are right about the mains frequency in different countries 
in the past.

However, my guess is that at least from the 1960's or so the mains frequency
has been kept rather close to the nominal frequency. You could see this from
electric clocks that used mains electricity. They rarely needed adjusting.

The electric companies try to have the same mains frequency after 24 
hours from
here. The frequency could creep one way or the other in the previous 
but it was slowly speeded up or lowered during the day to catch the 
right frequency.

The tiny variation of the mains frequency has been used in forensic audio to
define at what time on a certain day a recording was made. As the electric
companies have logged their frequency variation, this has been possible.
As far as I know, we are talking about very small variations here.

I have no idea how stable the mains frequency is today.