GM4 file extension is for Autologic mode 4 gray-scale files Bitmap graphics
Similar to GM4
#24 - LocoScript Printer data file for 24 pin matrix printer
#GF - MetaFont Font file
#IB - LocoScript Printer data file
#SC - LocoScript Printer data file
#ST - LocoScript Standard mode printer definition file
$$P - OS/2 Notes file
$$S - OS/2 Spreadsheet
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