Early livery button with ducal coronet and monogram M J R

A very unusually early silvered livery button with the monogram  M J R beneath a ducal coronet. The open backed 25mm button has the backmark WILLIAMS & SON. LONDON and it is considerably more domed that the vast majority of livery buttons. Of the hundreds of livery buttons we have this is one of only a very few we can confidently date to the very early 19th century. Although we have not been able to identify the initials I recall only one ducal family name beginning with R, that being the Russell of the Dukes of Bedford.    sep19/1