Prints on pottery are rarely the product of a potter's or an engraver's imagination. There is usually a source design which the pottery manufacturer thinks will make a profitable pattern and which the engraver can use as a prototype.
Most sources used by engravers are prints, which in turn derive from portraits, paintings, book illustrations or document details.
Finding the ultimate source of a blue printed design is challenging. Popular prints were issued in many editions and over a number of years.
To see a visual index of source prints that relate to patterns click here