Your first big film success was A Fist Full of Dollars, whose score is very different from that of the Hollywood Westerns. Your cultural perspective is obviously different. What were the stimuli?
The stimulus was the film itself. Leone had made an ironic and, in a certain sense, a grotesque film in that it was funny, a caricature. It was necessary to respect the clarity that Leone wanted for his characters. Besides, I was not, and am not, a specialist in American folk music, so what sense was there in my treating the characters like Americans? If that is required, use an American composer. So, I treated Leone’s characters by attempting to re-invent, in my way, American folk, bearing in mind certain musical and technical data. And then the caricatured treatment of the characters encouraged me to introduce strange sounds into the score so that the character would have the charisma Leone wanted.
Not that I know much about it, but I find music in some old japanese movies very interesting, like Woman In The Dunes (https://youtu.be/UFbJ9iDeZgg) or Kwaidan (https://youtu.be/v_OFoOwzPZY)
BTW, the film music for both movies you've linked has been written by Tōru Takemitsu, one of the most important contemporary classical composers of Japan in the 20th century. He is also quite well known outside of Japan (at least in New Music circles).
That makes sense.
Another thing I noticed is that the harmonic and melodic language Morricone's early Italo Western music has striking similarities to instrumental guitar tunes of the 60s by artists like The Shadows (https://www.youtube.com/watch?v=EzgbcyfJgfQ) or Neil LeVang (https://www.youtube.com/watch?v=ir7fjfqzcOM).
Anyway, like most great artists, Morricone borrowed from various sources to create his own unique style. Even Mozart, whose style many people would nowadays describe as very "pure", was accused by contemporary critics of being too eclectic.
He was very well aware of the contemporary classical/experimental music scene and you can hear the influence in his scores.
Guess that makes "Once Upon A Time In The West" the one where he fused the US-American style with his own
However, I can see a similar approach in Masaru Satō's music for Kurosawa's Samurai movies with its sparse instrumentation and inclusion of jazz elements. The whole score is in stark contrast to the music of earlier Jidai-geki movies, which also favoured large sweeping orchestral music.
Given the fact that "A Fistful of Dollars" was basically a remake of Kurosawa's "Yojimbo", I have been wondering if Morricone had been influenced by Satō in any way.