I have now been in contact with Janusz and he has explained the implementation.
The Journal # in GL trans view is a column for transactions that are closed by 'Closing GL Transactions' in Banking & General Ledger.
We have changed the column header to GL #. It will show 'None' for inside a non closing period and a serial number for inside a closing period.
Some small adjustments have been done. Committed to Repo.
File /includes/ui/ui_view.inc can be downloaded here
File /gl/view/gl_trans_view.php can be downloaded here
File /gl/inquiry/journal_inquiry.php can be downloaded here.