lbdb_dotlock(1) on Linux
The text 2e2b7fade363, as Debian 8 (Jessie), Debian 7 (Wheezy), Debian 6.0 (Squeeze), Debian 5.0 (Lenny), Debian 4.0 (Etch) shipped it.
Lock mail spool files.
Flags it lists
-
-d -
-f -
-r -
-t -
-u