Copperplate engraving St. Peter's Basilica

We have been researching for a while and have found very little regarding the artist and any past auctions for his work.  The only other places we could locate his architectural etchings are the Metropolitan Museum of Art (who I contacted this week and am waiting on the curator for the pieces to evaluate my images) and the Vatican Fabbrica.  The artist is Giacomo Sangermano, who was an architect working under Nicoletta Marconi during the restoration of St. Peter's in the mid-late 1700's.  There is a letter on the back, which I will attach an image of, from a business in London which is now closed.  Any help on this piece would be greatly appreciated!