At some point in time the "factory software version" field was added to the manufactured
devices, and when the device was manufactured without it, the value is blank.
Anyway, this field is only used to prevent installation of software older than that version.
Some people prefer to install a very old software version they once have tested and found OK,
but this approach does not work on MikroTik equipment because sometimes the hardware is changed
in a new production run and the software is modified to support it, and the older software does not
work on the newer hardware.
Apparently after this has caused difficulties a few times, this field was added and now the installer
refuses to install software with a lower version than in that "factory software version" field.
As long as you do not try to install old software, it is not important what value is in this field.
Thank you for this comprehensive explanation, now I understand completely.