Here is a colorful, annotated series of powerpoint slides prepared (and recently updated as of 20 May 2018) by Greg Bondar. Greg prepared these from research of TEI coding, our photos of letters from the Reading Central Library (posted with their permission), and information from the canonical reference tome, Alcock and Holland’s The Postmarks of Great Britain and Ireland. (Note that the online version of this is not complete, and we have the hardbound version to consult within the Mitford project.) Greg designed this series of slides to help us with identifying and encoding some of the more challenging aspects of 19th-century mansucript letters! Thanks, Greg!
The Digital Mitford’s Guide to 19th-Century British Postmarks, and How To Code Them in TEI