These were great models and unfortunatley not loved by many due to the big 1/43ish scale. I always liked this truck and the models big size, plus they were from my era growing uo.  Makes me want to dig out mine. The passenger side seat was modelled correctly by Dinky. A larger, more comfortable passenger seat was installed by Mercedes so the team driver could relax and sleep on long distance runs if needed.

Those were the displined driving total concentration days at Mercedes Benz which lasted well into the early 1990's. The thinking then was that armrests on the driver seat would have encouraged the driver to have his arms resting when they should have been both firmly on the wheel, They were one of the last vehicle builders to install cupholders in the late 20th century too and this was part of the same thinking from that total safety no destractions mindset.  

Here is a spec sheet  on the LP/LPS cab and it clearly shows the more plush passenger side seat with armrests in question.