Ueshiba claimed Shinkage-ryu as part of his influence -- Aikido Journal still holds forth that O Sensei was granted a Yagyu menkyo kaiden by Masakatsu Nakai in 1908 in Sakai City -- who reputedly was proficient in both Shingan and Shinkage. Many sources attribute Ueshiba's menkyo kaiden to him and assert it was Shingan:
Perhaps not -- or perhaps both.
OFCOL. I just said that he did Shingan --The point is that there have been references to Nakai also having Shinkage training as well as his main license in Shinkan -- maybe spurious, but they are out there -- which I simply added as a point of consideration to the apparent discovery of an actual Shinkage document granted to Ueshiba (whatever its provenance) that you hammered me for even suggesting from other evidence before.
Erick - there is NO evidence whatsoever that either Nakai or Tsuboi ever had any experience with Yagyu Shinkage-ryu. None. That some uneducated (in Japanese martial history, be they non-Japanese or native) people have read "Yagyu" associated with Nakai and/or Tsuboi and immediately assumed it must be Yagyu Shinkage-ryu is not evidence of anything other than their own ignorance. It's not even supposition. It's on the same level as "Japanese swords are called "ken," and my friend's name is "ken," so he must be a reincarnation of a samurai."
The "license," as Josh points out, is actually more evidence that Takeda was cavalier in his assertions of history, and simply gave a certificate because he could. Honestly, I wouldn't be surprised if he was nagged by Onisaburo to give Ueshiba something and he just decided that "you want a certificate, here's a certificate." That he didn't even name the 10 generations of Aizu's (non-existant) Yagyu Shinkage-ryu swordsman including his own teacher (that would be quite an insult, actually, were such a man to have existed) is further evidence that this is a useful "fiction."
If you or anyone else finds this implausible, I have an eyewitness account from Terry Dobson where a man who later became quite prominent in jujutsu circles in America, approached Ueshiba, crying, saying that if he went home without a black belt, he'd be shamed, and Ueshiba patted him on the back and said, in effect, "Don't cry. Here's a shodan."
I'm not saying that Ueshiba did the same to Takeda - although I wouldn't put anything past Onisaburo - but the LEAST plausible explanation is that Takeda held a secret inkajo in Yagyu Shinkage-ryu. Equally implausible is that Ueshiba's Yagyu Shingan-ryu teacher had a secret Yagyu Shinkage-ryu certificate (really, you need to get out more - even on YouTube - and see that the two ryu are really quite far apart.). And finally, since we already know that Gejo taught Ueshiba Yagyu Shinkage-ryu kata, why strain after explanations when it's right in front of you?
I should be more humble, perhaps - but I will note that I have an entire section in HIPS on Yagyu Shinkage-ryu influence on Ueshiba.
If there is any issue left, it lies in people actually familiar with the gokui of YSR (physical, moral and spiritual) assessing if Ueshiba evidenced any familiarity with YSR beyond a few kata which he "aikified."