in fact this is almost certainly Heinz Grimm trying out Günter Hildebrandt's eigenbau car out towards the end of the 1952 season, pending driving the car the following year.
this particular picture revealed the unusual position of the 500cc BMW boxer engine as side on, with one barrel facing forwards and one backwards.
As Günter Hildebrandt owned a Manx Norton it was thought that was the engine until this picture came light.