Нашел у STM AN UM0424, там что написано про 12 digits... In all available demos in the “STM32 USB-FS_Device developer kit” there is an example
implementing a unique serial number string descriptor based on the STM32 Device Unique
ID register (12 digits).
The default value of the serial number string descriptor is “STM32” and during the USB
initialization the Get_SerialNum() function reads the Device Unique ID register and sets
the serial number string descriptor.
а вот STSW-STM32121 найти никак не могу