I just know I'm going to get shot up here but here is my understanding:
Number on Vin plate;
Number on top of firewall (stamped in).
Number on engine block on LH side near bell housing;
These should all match.
You could probably take it further to include that the build dates on the head and block match the build details from BMW.
@Markos @Keshav @everybodyelse?
That do we think?
My name keeps getting thrown into the expert mix on threads as of late. I sure like to talk a lot and can type very fast, but remember that post count has nothing to do with experience or expertise.
I've never owned a classic car worth caring about matching numbers. I've only owned my BMW for about 4 years and I've driven it less than 20 miles.
I really only know how the car goes together, even though I've never put one together like many many other less vocal members have.
Disclaimer aside, I'm deviating a bit with my wall of text, but my two cents since you explicitly asked for it.
If I were buying a numbers matching e9, I would want all three (or four for US models) VIN's to match. I would want to see stamped VIN's, not engraved VIN's(which I and others in this thread have seen). If I were to buy a restored e9 or otherwise molested e9 (aren't they all), I would take condition of the car above all else. Once the playing field was equal, I would take the numbers matching car over the non.
CSL:
If I purchased a CSL, I would specifically seek out a numbers matching car. The folks who buy CSL's also buy other marks where numbers matching is important, and sell to buyers where numbers matching is important. Guilt by association. I would however take provenance over a numbers matching car. I would gladly take a CSL with provenance and an M49 in it.
. There are still many aspects of a CSL that make it a CSL besides the motor. You have the special seats, steering wheel, aluminum bits, road wheels, suspension, thin sheet metal, bumpers, etc. If your CSL isn't numbers matching or you can't find a numbers matching car in your price range, I wouldn't lose much sleep over it.
. I wouldn't buy a rough CSL over a nice one solely because it was numbers matching. The reason being is that restoring a CSL "properly" is Pandora's box. The sheet metal is different, the parts are rare, and everyone is ready and willing to judge the finished product. Buy a rough CSL because it is a good deal, or a fun project, or a sound investment.
Survivor:
If I found a survivor car that isn't restored, then numbers matching would be something to justify all of the other body and interior flaws that should be left original. I would ideally like the aluminum VIN plate on the fender to be original, with original rivets. I would do my best to back-date items that had date stamps. There are a lot of date stamps on the car, some visible and some not. The cylinder head, the underside of the valve cover, the coolant flange, the wheels (steel and aluminum), etc. It seems frivolous, but what else am I going to tinker with on a car that shouldn't be restored.
Buying Number Matching vs Keeping Numbers Matching:
These are two totally separate animals IMO. It does make sense to buy a numbers matching car, especially if it is of equal condition and price to it's peers. If someone is advertising a car as numbers matching, they probably want their special numbers matching price also. That is when I start to lose interest. I would rather find a numbers matching car in a barn, not on Hemmings. As far as swap on an existing car, numbers matching is important enough for me to keep the 2.8 block on my otherwise bastardized e9. I just see no reason to ditch the motor. It's not broken. It runs fairly well. I'll never really drive it that fast. If I am so inclined, I can eek more power out of it if I desire with the same mods that a B35 gets, the result will just be proportionately less dramatic. If I did switch to one of the three qualified replacements seen below, I would probably stick with the painful decision to store the short block. The single reason being - to appease the Porsche owners that chime in on BAT when I eventually sell it.
The funny thing is that nobody is going to spend thousands to have a mechanic remove an M90, B35, or dirty B34 only to install a 2.8. Nobody wants a 2.8 coffee table,. Nobody wants a 2.8 eating precious space for extra wheels in the garage. Nobody wants to maneuver their lawn mower around the rusty 2.8 short block in their shed. But this is what we do for "numbers matching".