psmask(1gmt) on Linux
The text 6b2ff0802377, as Debian 4.0 (Etch) shipped it.
To clip or mask areas of no data on a map
Flags it lists
-
-: -
-B -
-C -
-D -
-E -
-F -
-G -
-H -
-I -
-J -
-K -
-M -
-N -
-O -
-P -
-R -
-S -
-T -
-U -
-V -
-X -
-Y -
-bi -
-c