Kisshomaru reported that his father studied five years of Gotoha Yagyu Shingan under Nakai Masakatsu, from 1903 until he died in 1908. FWIW.
Don't have books here, so just some citations from aikido journal:
"Masakatsu Nakai 中井正勝
Middle school teacher in Sakai, Osaka. Teacher of Yagyu Shingan-ryu Jujutsu
with whom Morihei UESHIBA studied ... "
"The technical content of this school is unknown but certainly included jujutsu techniques and the study of various weapons. Records are unclear as to whether Ueshiba's direct teacher was Masanosuke Tsuboi or Masakatsu NAKAI. "
"Receives Yagyu-ryu Jujutsu