Prewar "aikido" is almost surely most accurately preserved in the Takumakai. The vast bulk of their material is preserved religiously from Ueshiba, with appr. 30%, as I recall, from Takeda Sokaku.
Not to divert the discussion, but how familiar with the Takumakai are you? Is the above statement still true in light of Okabayashi Shogen's influence?
Though Okabayashi has since broken off on his own (forming Hakuho-ryu, the style I study), while he was still affiliated with the Takumakai he was considered one of their more prominent instructors. After training with Takeda Tokimune, he was part of an effort to add the Hiden Mokuroku (and possibly other elements) from the mainline to the Takumakai curriculum. (I'm not sure about the politics behind that change, though I know Tokimune wanted to consolidate Daito-ryu before he died.)
Okabayashi's personal style certainly changed as a result of his time with Tokimune, but I'm curious about the extent and endurance of Okabayashi's influence on the Takumakai. Did the Takamukai become a bit more "mainline-ized" in their actual movements, or did they just re-structure their syllabus? I know that there was a backlash against the syllabus changes, but I don't know if that included removing the Hiden Mokuroku (and whatever other elements) after Okabayashi broke away.