My mischievous attempts to throw you clever people off the scent worked. A girl has to have some fun, after all !!
Those who guessed it was a door bell were right. It seems to be an empty doorbell casing where the push button is missing. I spotted it on my doorknocker photo shoot tour of Chinon.
I thought it was a shame that someone had ruined what probably was quite a handsome door by mounting a regulation letter box in the middle.
It seems there is sometimes no respect for the old buildings and they are spoiled for practical purposes, doing things the cheapest way possible, with no regard for aesthetics.