Well I never knew that! Thank you, Eric.
However I suspect there is a minor logic error in the page whose link you have provided. A freshly charged Lead Acid Cell will provide 2V. Three LAC's in series (as in an American car battery of that era) would be 6V. However to ensure that the car battery remained charged, greater than 6V would need to be provided by the car's charging circuit. Hence I assume that is where the "extra" 0.3V comes from.
I seem to have a memory that modern day vehicles, with 12V electrics, can have up to 14V supplied to the SLA battery and so any in-vehicle electrics are designed to be input voltage tolerant -- 10 to 15 volts, I believe.