Assuming they use the same serial number system for Pontons as Heckflossen, chassis number on these cars is coded. First 3 numbers being chassis/body style, second 2 numbers being transmission (auto or manual).

It makes sense to me that if MB was selling bare chassis to companies like Binz for ambulance, truck, hearse and other conversion then the chassis number would be incomplete, possibly just the last 6 digits. So we might not get much help there.

Frame, suspension, engine and drive train being MB make it legitimate. The rest seems to be grab-bag parts from "S" models or outright custom conversion stuff like doors with triangular corner wing windows, external hinges for the hood, etc.

Meanwhile there might be plaques, stamps and manufacturers marks in the usual places - driver's side door post, radiator support and possibly the firewall too.