This trip happened 5 years after the trip John and Paul had in 1961 in Paris, and “it was an echo of the trip John and I made to Paris for his twenty-first birthday” (Paul McCartney - Many Years From Now, Barry Miles).
In 30th September, 1961, John and Paul arrived in Paris. In September of 1966, exactly 5 years later, John took a break from filming “How I Won the War” to meet Paul in Paris. “It was an echo of the trip John and I made to Paris for his twenty-first birthday.” - Paul McCartney