An unusual pair of insulated pearlware sauceboats early 19th century

An unusual pair of insulated pearlware sauceboats early 19th century, moulded with a feathered rim enamelled in green, each handle with a square aperture at the top, minor chips, 21cm. (2)

The aperture on the handle accesses a hollow base beneath the bowl into which hot water can be poured to help keep the contents of the sauceboat warm.