In 6mm bores Remington started with (I believe) 1:12" for the 244 Remington. When they figured out that was dumb and tightened it to 9 whatever for the 6mm Remington I think they just went ahead and used that for the 243 when they chambered it.
Whatever they started with for the 243, I don't think they did what TA 17 said and slowed the twist at a later time.
I didn't think they did either, but I believe they started the 243 win off with a 1 in 10", like Winchester did in 1955..