Rob, I am not sure of a MB part number but if you use the Contitech that Waqas alluded to, their Online Catalog shows the AVX 10 X 925 as correct for 280 cars up to 12/68.
For 230s, it is AVX 10 X 875. Interestingly, the spare I carry with me is this size (from Ray) but the one on the car is a AVX 10 X 888. A little more investigation shows the 888 is standard on many BMWs. Starts to make sense given that my mechanic is a BMW specialist... Just wonder how many other Bavarian parts he's used! The added length has not seemed to cause any trouble in this case.
James
63 230SL