The cab section of the fire engine flips out to form the lower legs, which means that in vehicle mode it would house the upper legs - which means that it cannot be hollow to fit the Nebulan or human component as with other Autobot Headmasters. I'd say that HasTak were trying something new with Hosehead/Cab in terms of the design and transformation. Fire fighters do actually sit on the outside of fire engines when operating or accessing equipment (e.g.: water cannons, ladder etc).