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X3G (formerly S3G) goes back to 2009 and predates any use of a duty cycle with the M106 command. S3G (Sanguino3 Gcode) was actually developed by RepRap.org (e.g., Adrian Bowyer and crew) and was used for the generation 3 and 4 RepRaps. (Maybe also the pretty-much-never-happened Gen 5 RepRaps.) It made it into the MakerBot ecosystem since the Cupcakes and Thing-o-Matics used RepRap Gen 3 and Gen 4 electronics and the firmware was shared between the RepRap and MakerBot ecosystem. (Indeed, Zach "Hoeken" Smith, co-founder of MakerBot was had a title within the RepRap organization and significant involvement with RepRap.org.)