Kelly,
Sears never put a straight-vane agitator in a LadyK or the like, not from the late 50s on anyway. From the later 50s until 1965, LadyKs and higher-end Kenmores had Roto-Swirls and then Super Roto-Swirls. From 1965 through 1968 (ish) Lady Kenmores had the Roto-Flex. There were some odd-ball 1966 900 models that had a Roto-Swirl, but that's as "low" with agitators as they went. From 1968/1969 until 1974, Lady Ks had the adjustable, expensive Vari-Flex agitator. For the 1974 Lady, the Penta-Swirl was used (Kenmore's then best agitator). Beginning in 1976, all Lady Ks had the Dual-Action from then until the demise of the Lady Kenmore model.
Actually, looking back, I can't think of a single 29-inch Kenmore washer model that had a straight vane as standard equipment beyond the 60 series models. In fact, some 60-series and 600s from the 1960s to the 1980s had Roto-Swirls, Penta-Swirls, and Dual-Action agitators.
This varies a little bit with 24-inch machines. 24-inch models had a narrower tub than the 29-inch standard capacity machine, so the comparably narrow straight vane must have been judged by Sears or WP to be the best suited agitator for that basket size. All 24-inch models had the bakelite or polypropylene straight-vane, at least that I am aware of, up until the late 1970s when the standard capacity Dual-Action (which is also more slender than the other comparable KM agitators) debuted in the highest end 24-inch machines. There were a few very highly featured 24-inch machines, so it would have seemed a little odd to have a straight-vane in them, but they did. I will say this though, the Kenmore gold plastic straight vane may not look very impressive, but this agitator MEANS BUSINESS and does a very good job. On low water levels and normal speed agitation it can splash gallons of water out of the machine, but it is not going to mess around with no-turnover laundering.
Gordon