xbiff(1) on Linux
The text 1c5a70f4aeec, as Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie), Debian 7 (Wheezy) shipped it.
mailbox flag for X
Flags it lists
-
-bd -
-bg -
-bw -
-display -
-fg -
-file -
-geometry -
-help -
-rv -
-shape -
-update -
-volume -
-xrm