I'm guessing that this is the type of intermittent failure you could see in a fuel pump. Maybe?
BTW: What is a WUR?
Don't go throwing parts at it. New D-Jet fuel pumps cost more like $800. The one you have seen is possibly a rebuilt.
You need to either learn all about the Bosch D-Jet ignition and fuel injection system, OR try and find an independent that knows how to work on the Bosch D-Jetronic engines. (Early VW, Porsche, Volvo, Jaguar and others used similar system).
The engine for the '73 450SEL in this link
is same as yours - sections 7.4 and 7.5 cover injection and ignition. There is a section on diagnosis including trigger points, except they call them "release contacts" (Section 07.4 040-10 for M117 engine using standard voltmeter). The attached pdf calls them "sending contacts" ??
For general info, this link
may help, and I have also attached a troubleshooting pdf.
It could be that you may just have a loose connection to the trigger points or even a bad ground for the injectors on the top rear of the engine under the air cleaner.
Try to check everything systematically, starting with fuel pump (you can hear it run when you first turn key on, but only for a few seconds, then it cuts off until engine is started). As mentioned before, if you hear nothing, check the fuse first!