Powerline frequencies in major cities in the US were not synchronized until after World War II. At all. The citation for this statement can be found in the archive. It came from a US military source.
A much more detailed analysis of the vagaries of powerline frequency pre-1965 was made by Mr. Brock-Nannestad on Nov. 3, 2009, with the subject heading "power line frequency".
Hum signatures are a valuable resource for speed correction when they come from a synchronized source, such as the modern power grid.
> Date: Fri, 31 Oct 2014 06:23:03 -0400
> From: Tom Fine <[log in to unmask]>
> Subject: Re: Robert Johnson Wrong Speed?
> Hi Thomas:
> The 2011 reissue IS the correct speed, based on the 120hz hum from the original recording equipment.
> I was trying to stop this mythology, after talking with Seth. Let me say again -- the 2011 IS the
> correct speed, unless there was some freak power system present in Texas that operated at some
> frequency other than 120hz (just about zero chance of that). So, again, what you hear on that 2011
> edition IS the correct speed. All other discussion of "theories" of other speeds are not based on
> facts. To quote Seth Winner again, "the hum doesn't lie."
> -- Tom Fine