Prescribed ointment would have been put in and a piece of paper put over the top tied around with string.
It pre-dates the fire of London so a period of around 1650
I found it on the foreshore of the Thames in the early 70s just near Hungerford Bridge south side of the river
It was dated by the London museum and also the Victoria & Albert museum.. It is almost perfect just a slight nibbling of the rim normal for Delftwhare.. The museums at the time dd not have a complete one
Used to mudlark in my lunch hours when working in Waterloo... Have loads of other stuff