Emperor Hirohita's butt-boats (sort of)

I was recently browsing through a book about WWII, and found the amazing photo below.

It shows Japanese scouts using one-man inflatables to cross a river. Several things to note:

  1. they are designed so that you lie on your chest, feet behind you in the narrow part of the boat (so not really “butt boats”);
  2. instead of paddles, the guys have two one-hand paddles that look a bit like ping-pong paddles–I guess you did a “butterfly” stroke;
  3. the cut of the bow in the closest boat looks a lot like our good friends the alpackas!

I don’t have any more information about these boats, but I’ll give you the reference to the book and the caption that it provides:

“World War II Combat Reconnaissance Tactics”
by Gordon L. Rottman, illustrated by Peter Dennis
Osprey Publishing, 2007
ISBN: 978-1-84603-137-3

Caption: “Japanese scouts demonstrate beaching on a river bank using one-man pneumatic boats; these were provided with hand paddles and could only be used on relatively calm water [ed–sez you!], but the scouts wear kapok life-vests just in case.”

I wonder if these were made of silk, with natural rubber coating? Possibly an off-shoot of balloon technology?
Clearly an important chapter in the Complete History of Butt-Boating!
(sorry about the photo quality: I snapped it with my Mac’s built-in camera, but the original is not much better).