https://www.awm.gov.au/collection/C2135093
The 'beetle back' pattern was so named because of its distinctive sloping rear section, manufactured between 1938 and 1944. The unique shape was derived from a design to make the vehicle easier to decontaminate in the event of a gas attack. The 'beetle back' Quad is believed to be the only vehicle to employ this design feature.
from Australian War memorial web site