Why 4 sleeping places but only 2 seats in motorhome (James Cook, Bremen Transporter)?

Hello, only two seats are registered in the Mercedes 207D car registration from 1979 (T1, Bremer). Why? The camper (James Cook, i.e. ex factory) has 4 sleeping places – should the other two passengers run next to it? At that time there was no belt obligation at the back, why are not at least 4 seats registered? Greeting Stefan