As far as LK8000 (XCSoar spin-off) goes, the "recorder serial number" is
not your IGC-approved FR number, but a random number LK8000 will use to
mark it's files. It's important on competitions where LK8000 logs are
accepted, so two pilots do not have same serial number in their files.

AAA is not a good choice, put some random letters or something personal
to you.

With 302, you can check serial from any file it has generated. Open it
as a text file and study the first line.

--
Wojtuś