In the Kloss library there is a book with rituals ascribed to Baron von Löwen. On the tracing board for the degree of “Parfait Macon”, there is a drawing in which I see nothing. It is very prominent, so I suppose it refers to an element of the ritual.
I wonder if it could represent a grave, especially when I see this:
Top: Collection (1844) of Georg Kloss (1787-1854) (Collection de 84 tableaux, Kl.MS:XXV.1)
Bottom: Feddersen (Die Arbeitstafel in der Freimaurerei Band I (1982) SD/8