According the the Spode Museum Trust's page on the Frog pattern: "Frog pattern has been produced in various versions from about the 1820s. No one really knows why it is called the Frog pattern and the name may have originated as a factory name - if you look at the outside of the pattern the shape is similar to a flattened frog!" The "Imperial" in the mark stands for a special earthenware body.
This plate is 9-1/4" in diameter. It is gadroon shaped. It has no chips, cracks, crazing or repairs. There is a little wear to the interior edge from being stacked over the years.
It is marked in blue print: Spode's Imperial, which is Godden's mark 3656. He says the mark was used from 1805-33. But, if we go back to the Spode Museum Trust page we have this description of the pattern: "4233 first recorded c1826, gadroon shape, printed in blue and coloured over in puce and gilded." In the book, "Spode Printed Ware" by Drakard & Holdway, they say the mark was used about 1824-33.