Giovanni Battista Piranesi and workshop

Study for an etching in "Diverse maniere d'adornare i cammini," 1769, f. 893.
This drawing is an interesting and complex work, different in character from most of the rougher sketches associated with the Diverse Maniere. It is constructed of two sheets of paper, and the monopods, particularly that at right, are not drawn by Piranesi himself, although they might be attributed instead to Nicolas François-Daniel Lhuillier, as Christoph Frank first suggested. It is as though Piranesi took an existing drawing of the monopod by Lhuillier (the right half of the sheet) and had Lhuillier or some other assistant copy it in reverse for the left jamb of the chimneypiece; after the sheets were joined, Piranesi himself seems to have drawn, with a thicker piece of red chalk, the architectural framework and a first sketch of the ornament on the lintel, which he then refined with pen and ink.
The monopods could have been based on any of a number of ancient remains (or modern copies of those) in Piranesi's workshop: an alabaster example from Piranesi's collection that was sold to the Vatican Museum after his death (now in the Sala degli Animali), or another which Piranesi sold to Charles Townley through Thomas Jenkins in 1774 (now in the British Museum), or those of the tripod sold by Francesco Piranesi to Aubrey Beauclerk in 1778, recorded in the Vasi, Candelabri as plate 72. At a certain point, however, it becomes difficult to know whether (or when) Piranesi had an actual object in mind, or just a drawing of it.
The wall panel sketch on the verso is probably related to Piranesi's work at Santa Maria del Priorato; it may be an early design idea for the wall enclosing the piazza outside the complex.