According to Karl Friday (menkyo kaiden in Kashima Shinryu), there's no evidence supporting Ueshiba's ever studying KSR. There are some folks who have done so in the Ueshiba lineage however, and there's some debate about whether or not they're actually qualified to be teaching KSR, but that's a whole 'nother cuppa fish.
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I've heard of some connection between the Ueshibas and the Kashima Shinto Ryu, however, but have no firm info on that.
According to his own words in "Aikido Ichiro", K. Ueshiba was ordered by his father to study Kashima Shinto Ryu with the 63rd headmaster of the ryu. According to Meik Skoss M. Ueshiba made keppan (blood seal) with Kashima Shinto Ryu, but didn't actually train there, sending some of his students to train instead.