Member, P500 Sport, DSC off (>900)
Join Date: Oct 2009
Location: New Zealand
Thanked 179 Times in 139 Posts
Not sure about the ////M models, but it is very common for BMW's of the 80's era to have the same engine number as the last 7 digits of the VIN.
I've owned several non-M E28's, which all had/have matching numbers.
Here's the details of my current "project" as an example:
Model: 525 EE
Main Colour: White
Engine No: 1866647
So it would not surprise me at all if your early M635CSi has similarly matched engine/VIN numbers.
1987 E28 M5 (DC98) Delphin-metallic/Midsilver Nappa
1986 E23 745i/M88 (6928) Arctic-metallic/Oyster Nappa