If the building was built before 1840, then any letter box would be a retrospective addition. Therefore check if installing a Letter Plate is suitable for the external door.
An external box for letters may be more appropriate to retain the period appearance for Georgian properties.
The earliest were of plain cast brass, without any decoration, often fitted vertically on the centre line of the door.
There was also a narrow roll-edged type. Early examples usually have a small opening which is unsuitable for modern mail.
The configuration of the door should dictate the size and location of the letter plate.
Letter plates can be fixed horizontally on the centre line of the lock rail of a single door.
Where there is a single door with central beaded muntin, or a pair of doors, the letter plate should be fixed in the centre half of the lock rail opposite to that of the doorknob.
Letter plates would probably not have been fitted until the introduction of the penny post in 1840.