STM32F4 USB serial number not unique!

On the  different STM32F4 Discovery boards with STM32F407 that we have here the serial number of the USB Virtual Port connection, as reported on the PC, is always 00000000001A.

Is this as it should be? Also, strange low number.

We also have another stm32f4 system with does have a 'real' serial number.

The actual question is how can I distinguish between usb connections when more systems are connected.

I need this to create a unique device name. This is in Linux, and udev rules are used to do this.

Normally I use the serial number for this, but what can I do now?

  Thanks in advance,

       Sietse