Unusual for its time, similar to 'Memento'
Both movies are about a man with memory loss who witnessed somebody's death, and both have repeated scenes/flashbacks.
It's a 1960s film but the structure and editing make it feel modern. A lot like something Christopher Nolan would make today (he wrote/directed "Memento" and "Inception"). If he made films in the sixties, they would be like this. I wonder how audiences in 1965 reacted.
I wouldn't be surprised if this was an influence on Nolan.