I rewatched this yesterday for the first time since the 70's, and was startled to hear some of the score used in Romero's 'Night of the Living Dead.' Two films from the same year - what's up with that? Was it library music?
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I don't know about The Bamboo Saucer, but Night Of The Living Dead is known for having used library music from the 1930s and 1940s in an efficient way.
So what you say might be possible.
"Being lost in a film is totally fine."
Theres at least one piece early on that I recognized from other movies, but cant place what.
Probably stock background music.