Most noticable are the number of diamonds on the cargo side.
The normal AHN has a flatbed with 5 diamonds, the gasifier AHN (AHN-H) has 4.
The normal AHS2 and AHS3 has a flatbed with 4 diamonds, the AHS4 and the gasifier AHS (AHS-H) have 3.
The normal AHR has a flatbed with 6 diamonds, the gasifier AHR (AHR-H) has 5.
All platforms being 1 diamond shorter for the location of the gasifier.
A second difference between AHS and AHN is the height of the bumper. (see photo)
A third thing is the low hinges of the doors.
On the AHN the door hinge is higher than the wheel wings.
On the AHS the door hinge is at the same height of the wing.
A fouth difference is the wheel size. The AHN has bigger wheels.
The AHS2 has steel disk wheels, the AHS3 and AHS4 have cast wheels like the AHN bus with smaller tyres.