I've been doing some more investigative work. According to Wikipedia the W115 series made before the W123 series also had a 5 cylinder engine. So I now have a slightly modified question:- would the numbers that I found be a part number for a W115? => 115 351 32 08

It seems quite likely to me as the "friendly dealer", from whom I bought the car, showed me all the new pipes he had fitted at the back of the car! Are the drive train parts for a W115 easy to stick onto a W123?

If does indeed turn out to be a W115 part would I be experiencing the 7% discrepancy in speed that I am experiencing or should I indeed be looking at the speedometer?

As a point of interest at highway / motorway speeds I'm scared to go any faster than an indicated 85mph (about 140 kph). I don't know what the engine revs are at this speed as I have no tachometer but it just feels like that is the safest speed to expect from the car although I have a feeling I could "do a little more"...