I hadn't noticed this is the same problem, but indeed. In fact the OD1P - Aug 10 Problem is an improved version,
Looking back to the book, I noticed that Hashimoto Utaro added the marked stone to the problem. He marked it too, implying that there is some comment about the stone.
The white stone at a prevents Black from running away, which would provide another solution.
I have had a brief mail exchange with Richard Hunter who possesses a copy of the Gokyo Shumyo with solutions and comments by Go Seigen. Go Seigen made several improvements to the previous versions, precisely preventing the running away options.
The "improved" version can be found on several Chinese websites. Ulrich Görtz has a web page for these classics, converted to SGF at http://www.g0ertz.de/uligo/classic.html.
In the Chinese version Qi Jing Zhong Miao problem 26 and 30 are completely different from your version. Who knows why?
The funny thing is, I have this collection on disk. Download but never look at it: that's called greed.