CadStd (CAD Standard) is a CAD program by Apperson & Daughters, available in free and premium versions. It will export data in a number of formats, including DXF, SVG, and HP-GL, but has a native format with a .cad extension.
Files appear to begin with the bytes
43 41 44 53 54 44 (hex), which spells "CADSTD" in ASCII.
The freely downloadable version of the program comes with some sample files. In a standard Windows installation, they can be found in
\Program Files (x86)\Apperson\CadStd\cad.