The scene was certainly done live, the only image I found from the excavations is a painting by Anton Schranz, this was certainly done before the painting, it is indeed missing certain parts which will later be brought out in the excavations.
The quality is very good,
On the back stamp of the Contessa Rosa Piatti Lochis Crocetta collection
The watercolor may have been cut in ancient times
tear restored on the left, right, small collector's stamp