The 'mystery' prints featured in ATG No 2500.
A possible explanation is that the prints were published by Louis-Marin Bonnet, who in 1774 started publishing prints with a faked English origin in order to add gilding to the prints. In 1776 he opened a shop, Au Magasin Anglois.
The gilding on prints stopped in 1777 but at a guess he continued to use the deception from time to time.
This information comes from catalogue entry 30-34 in Colorful Impressions from the National Gallery of Art, Washington 2004.
George West
