Above under tray replaced by an equivalent in fiberglass

Soon after the introduction of the 2800CS and the conversion by Alpina and Schnitzer of some of the cars to compete in competitions, feedback to Munich was that the CS shape generated quite significant front end lift at speeds above 100kmph. The company realized that this was a safety issue and introduced a compressed fiber under tray which considerably reduced the lift. For competition Alpina took the solution further by introducing a front lip spoiler to reduce any remaining tendency for lift to be generated.
The part is still available in 2016 and BMW Classic have taken the opportunity to upgrade the original compressed fiber to a more durable plastic part number 51 47 1 811 948 costing about €220. The original, although it was a flexible board impregnated with a waterproofing, only had a normal service life of about 3 to 5 years and although it did its job reducing front end lift it was not really fit for purpose.

30 SI seem to have had the same/similar under tray.

Credit / Thank You / Reference: John Castle