[gilles collaveri]

Let me share with you the information I received. It completes very well what we already received:

The AP / SUM button comes from an "Anlass-Pumpe" (engine starter pump) designed and manufactured by the company SUM (Sum Vergaser-Gesellschaft Carl Wirsum & Co. KG in Berlin) originally a supplier of carburettors, but then also of other fuel system and engine components for the German Luftwaffe.
The pump was operated manually by the pilot (or the engineer). By pushing the button up and down, a specific starter fuel (mixture of a special volatile fuel 96% and 4% oil) was pumped and sprayed into the engine to ease the starting procedure. The pump was integrated into a small tank for the starte fuel;

Cheers

Gilles