I'm pretty sure the Dual Action agitator came out around mid '70's( 1976 or '77). I think there were 2 sizes of Roto-swirl agitators. One for the standard tub and one for the larger tub. Since it was such a "revolutionary" design that you didn't know you needed, Kenmore offered it in mid-line & up models. It seems like they dropped the straight vane & roto-swirl in favor of the penta-vane & penta-swirl in the larger tub machines. Funny how Whirlpool branded belt drive washers always stuck to the Surgilator. Only in the dd washers did they start putting a dual-action agitator. Roto-swirls were interchangable when they went to the lower centerpost(I think). Earlier they had the long centerpost with the bigger tub, hence needed a longer agitator.