The smallbox is not directly patterned after any original BMW Motorsport plenum to the best of my knowledge. More likely it was developed by a carbon fiber manufacturer in more recent years. It allows the user to convert to an Alpha-N setup while still retaining the original aluminum trumpets. This is shown more clearly in the photos below.

There are two features of the smallbox which may be undesirable. First, the airbox must be joined to the aluminum flange on the factory trumpets. The joint at this interface must be sealed, and this will not be as easy as with the stiff and flat aluminum factory plenum. Of course having a perfect seal is not nearly as important with an Alpha-N system as it would be with a standard AFM or MAF system, as a small leak of extra air coming in will not disrupt the engine management process.

The second undesirable feature of the smallbox is that the inlet to the trumpets will end up facing the opposite side of the interior of the carbon plenum with little room remaining for air to flow smoothly into the intake tracts. Some German tuners apparently feel this is a noticeable impediment to the power potential of this airbox relative to the Gr A and DTM style airboxes.