12M file extension is for Lotus123 Smartmaster file

Similar to 12M

096 - 96x96 dpi display font Ventura Publisher
0B - Printer font with lineDraw extended character set PageMaker
1 - Unformatted manual page Roff/nroff/troff/groff
10X - Gemini 10x printer graphics file - Bitmap graphics
123 - Lotus123 Data file
12M - Lotus123 Smartmaster file
15U - Printer font with PI font set PageMaker

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