The operating system of RevPi Flat is a (customized) Linux based operating system from RaspberryPi foundation. Those operating systems are called, for example Buster. It is important to specify the code name of your operating system when exchanging information with other RevPi users.
Starting from the “Jessie” version, you can retrieve the code name and release date of the operation system at delivery:
In this case it is a-Buster system from April 22, 2021.