Bert,
I see you have a 3.5 w/manual..if there is a chance of a 3:69 , that would be it. That is great.
If you look at the diff case from the left side, you will see a circle of bolts that hold the left axle tube to the diff case/pumpkin .. Right in front of the two bottom bolts, there will be a machined flat surface on the diff case about 2" long.
If you take a razor or sandpaper to that flat, you wil see the diff. serial # and then the last 3 digits will be the ratio....it will be with a comma, as that is what Euro uses for ratio, instead of the : mark [ in other words ..like 3,69 instead of 3:69 ]
It can easily be seen from under the car, as it is toward the bottom and facing the left/rear wheel.
http://www.sl113.org/forums/uploaded/ja17/2007427222029_gear%20ratio%20location%20late.jpg
If you do see it to be a 3:69, the other important detail is does it also have the oil cooler tube from the left axle tube to the case ..that will eliminate the possibility of the rear having ever been changed to the standard 3:69
case w/o tube. That will verify that the 3:69 ratio was in fact available in both cooler and standard differentials.
Many Thanks for your help.