Here are a few more things we can do with the Google Books’ Ngram analysis and Musashi.
Breaking out the British English from the American English, we can see there is no 1950s Ngram uptick in Britain, lending further support for my idea that it was caused by Samurai recognition at the Oscars, a very American institution.
On the other hand, we can see that there was a British bump in the late 60s and early 70s. What could that be? I have no idea. I can’t find a release date for Samurai in Great Britain, so it might the same as the USA. However, it might also be the same as West Germany, which was in 1965. Could there be a connection? What happened in West Germany? And what about in France with its release date of 1993? Let’s examine the Ngrams for western Europe. Here we see Germany in blue, France in red, Italy in green, and Spain in orange:
No, I can’t find any connections here.
The last thing we can do is to compare Musashi’s appearance in fiction with his appearances in all publications. As the Ngram shows, Musashi’s fiction:non-fiction ratio was highest in the 1980s (when the Yoshikawa novel was published), but his mention in non-fiction has been steadily gaining since then.