One other difference between the 1964 70 and 63 600 was that the 70 had the infinite water level selector, while the 600 had 3 designated levels where you could not set the selector between. It really offered no advantage as the tubs were not large enough where there was much height difference in the water line from one level to the next. Why Sears reverted back to the manual filter for both the 1964 and 1965 basic model 70s I do not know. (I say basic because 1965 also had a slightly more elite 70 that was an alphabet model with the self clean filter.) It wasn't until 1966 that the basic model 70 was outfitted with the self clean filter again. Incidentally in 1966, the model 600 series had a Super Rotoswirl, and was almost identical feature-wise to the 1964 and 1965 model 70s, with the exceptions of having designated water levels and the speckled tub. I can go even further with model evolutions, but I have been more long winded with this post than I had planned.
Have a good one,
James