Thanks for the info, Jay.
I have a theory about the 'Iga-ryu' that Hatsumi-soke describes in Essence of Ninjutsu. This theory of mine has been influenced by what I have read in Essence of Ninjutsu and Mats Hjelm's very informative articles concerning ninpo history, as well as my own speculation. Here goes...
I believe that the Iga-ryu that Hatsumi-soke is talking about is not actually a specific ryuha. In one of his articles concerning Iga-ryu history, Mats Hjelm seems to identify the 'grandmasters' of this Iga-ryu NOT as soke (head of a ryuha) but as jonin (head of a ninpo organization or network). I believe this 'Iga-ryu' was more like a collective of specific martial arts lineages developed and used to protect the people of Iga. In this regard, the Bujinkan Dojo could be viewed as a modern-day incarnation of this 'Iga-ryu'.
Thus, this 'Iga-ryu', this broad collective of martial arts within Iga encompassed a large diversity of ryuha at different points in time: Koto-ryu, Gyokko-ryu, Kumogakure-ryu, Togakure-ryu, Gikan-ryu Gyokushin-ryu, Hakuun-ryu, and Kukishin-ryu all being notable examples. All of the ryuha of the Bujinkan seem to be Iga-ryu schools, although I am not so sure about Shinden Fudo-ryu (does this school have connections to the Fudo-ryu school of ninjutsu?) or the Takagi Yoshin-ryu. Almost all these schools can be traced back to having been developed by a relatively small group of men (Ikai, Gamon Doshi, Garyu Doshi, Kagakure Doshi, Hakuunsai Tozawa, Iga Heinabe Yasukiyo, etc etc.), and thus have a common origin and culture.
There is an interesting point about the Kumogakure-ryu that Mr. Hjelms makes: "[...] IGA Heinabe Yasukio seem to be the basis of the knowledge that IGA Heinai Zaemon Nojo Ieanaga, his descendant in the 12th generation, used when he founded a school, also named IGA RYU, but which later had its name changed to Kumogakure Ryu Ninpo [...] Another interesting point worth a note is that Iga Ryu and Kumogakure Ryu who is listed among the single Ryu, is in principle the same Ryu. Since Iga Ryu changed its name to Kumogakure Ryu."
So, it seems there WAS a specific Iga-ryu martial art, but that this had its name changed and is now the modern-day Kumogakure-ryu.
In other words, there was the Iga-ryu as a unifying network of ninpo traditions, of which Gamon Doshi was the first jonin, and there was the Iga-ryu as a specific martial arts school, which later had its name changed to Kumogakure-ryu. And, in a certain sense, any martial art ryuha from Iga could always just call itself an 'Iga-ryu'. So, it's very tricky when trying to define what one means by THE 'Iga-ryu'. Regardless, I think the 'broad collective of ninja ryuha Iga-ryu' is the Iga-ryu Hatsumi is talkin about in Essence of Ninjutsu.
What do you think of my theory??