My 1st generation SAA has a bore that is .451". That said, the barrel had been replaced with a late 1st/early 2d generation barrel. I think the "early 2d barrel" is one and the same as the "late 1st barrel" since the stock of late 1st barrels was probably just being used up.
As to a date, I don't know. I have yet to find anyone who can definitively tell me when the change took place. I hope someone here can answer that for both of us!
I've heard that the change took place during WW1, and had something to do with the adoption of the .45ACP cartridge in the military. Around the 1910s the rifling was changed from the "black powder type" to having equal width lands and grooves. Perhaps that also marked the change in bore size.