Lawn auger are general purpose design for use in a variety of soils. Steel blades are riveted to tough malleable iron yoke. Steel shaft with a welded steel tee handle. Available in 2, 3, 4, 6, 8, 10 and 12inch heads.
The AU-S line. has the following pipe sizes:
AU-S2 extender uses a 1/2in threaded pipe.
AU-S4, AU-S6, AU-S8 and AU-S10 uses 3/4in threaded pipe.
AU-S12 uses 1in threaded pipe.
No extenders are stocked because it is so much cheaper to just go to the local hardware store and buy the correct size pipe pre-threaded. You would also need a female to female threaded adapter to connect the extension to the T-handle.