.123dx is 123D Design Model File
Key Features: .123DX File Extension
What is an .123DX file?
3D model created by 123D Design, a program used for designing models for print or fabrication; stores the 3D geometry of the model, as well as materials that define the surface appearance of the model.
123D Design models automatically assign connections between parts that give the model natural motions when moved.
NOTE: 123D Design has been discontinued.