Schmalspur-Modell-Forum

The last coach looks like it only has a single axle

Yes that's correct. The Mixnitz system had two off Ci-s with 3 windows and a wheel base of only 1,8 m or so which had later been extended to about 2,4. There war a D-s too with very short wheel base.
You will find the small drawings in "Schmalspur in Österreich" under this system from Styria.
best regards from Vienna
Josef