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There is a design difference between the after-1987 model radiator and the earlier model. This also affects the coolant expansion tank and the water pump.
Perhaps the most visually obvious change is that the coolant overflow tank was moved from the passenger side of the engine compartment to the driver side. Along with that, many other design changes were also implemented.
In addition to this, there is also a basic difference between the radiators for cars with automatic transmissions vs. manual transmissions. Automatic transmissions are highly sensitive to the temperature of the automatic transmission fluid. When this gets too hit, the automatic transmission can fail very quickly (as in, within minutes). Keeping the fluid cool is a good idea, and to do that, BMW routs the automatic transmission fluid through the radiator so that it get cooled in the process.
That’s why, when you buy a radiator, it’s necessary to specify whether your car has an automatic transmission or a manual transmission. The latter doesn’t need its fluid pumped through the radiator.