It's a difference in tempering.
Unfortunately, my knowledge ends at the Google search. Sorry I can't be of more help.Sorry, I stated my question badly. I had read the above link, when I look at ASTM B211 on the chart for 6061 it shows the Tensile strength, Yield Strength, and Elongation to all be the same between T6 and T651, however, T651 has the notation E which states "For stress-relieved tempers, characteristics and properties other than those specified may differ somewhat from the corresponding characteristics and properties of material in the basic tempers".
I am trying to find out what the differences are, or is it just diametrical tolerances.