Don't know Ezra, but I'm in Seidokan. My only comment would be that the late Rod Kobayashi was Chief Instructor for the Ki Society Western US up until he left to form his own organization (Seidokan) in 1981. So he would have some influence to varying degrees on quite a few people, at least here in the Western US where Tohei was tremendously active bringing Aikido from Japan to here. And of course Kobayashi also trained some under Tohei as well as the old-timer first generation students of Tohei in Hawaii like Yukiso Yamamoto, Kazuto Sugimoto, and Isao Takahashi. So like anything else there's destined to be those who followed similar threads and paths.
Also, Seidokan has always been a rather "large tent" organization with senior students of Kobayashi (and now their senior students for that matter) all over the world. Some of us like to visit other people too.
So he had quite an influence on a lot of people prior to forming Seidokan, then influence after the fact, then a lot of rather talented folk fanning out all over the world. Not to mention the fact that most in mainstream Ki Society will obviously have many similarities depending on their focus.
Just fwiw. Not sure if it helps.