A Lotus 123-1A document; can be read on any machine with Lotus 123 installed (available only for WinTel boxes, Macs, and some UNIX boxes, but usually not for free).

Spreadsheet Lotus 1-2-3 Version 1a

Spreadsheet files produced by Lotus 1-2-3. Later versions of Lotus had files with wk1, wk2, and wk3 extensions.

Well Known Service.