It has just occurred to me that perhaps not everyone knows how to find the part numbers on the parts.
First things first. The chassis number or VIN. For model identifiaction purposes you need to find the Mercedes VIN - I understand that in the US there is sometimes a US VIN. This is of no use to you if you want to extract information from the PDF file I've posted above.
An example of the "good" chassis number / VIN is
WDB 123 130 blah blah blah (where blah blah blah will be more numbers)
From the PDF in the first post you can see that the 123 130 blah blah blah chassis numbers are for
(W123) 300D
BUT you can also see in the same file that there are several other 300D variations out there. So sometimes someone might be selling something from a "300D" but it won't necessarily have been fitted as a standard part on
ALL 300Ds...
...this is partly the reason why I've compiled this information.
To find the chassis number on a W123 you can find it in the following places. Please note these places are sometimes different if you've got a true Euro model (not just the headlights!) or a US model.
On US models sometimes you'll see it on the windscreen on the left hand side
On all models you should be able to find a chassis number stamped into the bulkhead / firewall in the engine bay.
Engine numbers tend to be on the BLOCK stamped on the left hand side of the engine - here left hand side is taken as though you are sitting in the car looking forwards.
Transmission numbers - should be on the right hand side of the transmission (again sitting in the car looking forwards) just above the oil pan.
Note about differentials - you are unlikely to find a part number stamped on the casting(s) that compares favourably with the part numbers in the PDF. You're more likely to find a number for the casting. However you can more easily find the final drive ratio by scraping a bit on the back of the differential as shown in the picture below.
(Picture borrowed from DeliveryValve who kind posted it here in this thread
Differential identification? )