I'll confess to having an advantage on this one, having worked many years in Colchester. The Colchester Oyster Feast is annual event that has been celibrated for hundreds of years with various dignitaries and celebrities invited as guests. I suspect this was made as a special serving piece for the event.
Yes, this piece is directly related to the Colchester oysters. My book refers to it as an "oyster gauge". "No oyster offered for sale from the Colchester beds was to be of a smaller size than this official gauge."