wk1

from The Free On-line Dictionary of Computing (8 July 2008)

wk1

   <filename extension> (worksheet) The {filename extension} used
   by early versions of the {Lotus 1-2-3} {spreadsheet} program
   including release 2.01 (1987).  All files created by the user
   are given this extension.

   (1995-11-28)