440 is actually an international standard, not American.
>In 1939, there was an international conference held in London that resulted in a recommendation to use A = 440 Hz, as a compromise between the various tuning systems used at the time, some of which reached beyond 450 Hz. This recommendation was further supported by the fact that the BBC required their orchestras to tune to 440 Hz instead of 439 Hz because 439 is a prime number, and the corresponding frequency was hard to generate electronically with standard electronic clocks. Eventually, in 1955, the standard A = 440 Hz was adopted by the International Organization for Standardization (ISO).*
When I shopped for my piano, in Vienna, it was tuned to 443 which was the custom there. It was crated up at the factory and shipped to California where it's been holding its tune.
>In 1939, there was an international conference held in London that resulted in a recommendation to use A = 440 Hz, as a compromise between the various tuning systems used at the time, some of which reached beyond 450 Hz. This recommendation was further supported by the fact that the BBC required their orchestras to tune to 440 Hz instead of 439 Hz because 439 is a prime number, and the corresponding frequency was hard to generate electronically with standard electronic clocks. Eventually, in 1955, the standard A = 440 Hz was adopted by the International Organization for Standardization (ISO).*
* https://jakubmarian.com/the-432-hz-vs-440-hz-conspiracy-theo...