Sources of Information
Apart from examples of postmarks (by email to somersetdorsetphg@gmail, 300 dpi colour scans if possible please), Somerset postmarks have been gleaned from the following literature:
The Somerset listing of UK Post Offices by County maintained by Ken Smith
The Somerset section of the British County Catalogue [1992] as updated by the Somerset & Dorset Postal History Group (S&DPHG)
"Postmarks of Somerset, Steel Handstamps Proof Impressions 1826 - 1921" by Mike Welch, published by the S&DPHG in 1995
"The Climax Rubber Date Stamps of Somerset", collated from articles by Mike Welch published in the S&DPHG Journal
"The Undated Circular Marks of Somerset and Dorset", published by the S&DPHG in 1992 and updated by the S&DPHG
"Barred Numeral Cancellations of England and Wales" by John Parmenter, David Luckly, and Ken Smith, made available on the Great Britain Philatelic Society website
"Collecting British Squared Circle Postmarks" by Stanley F Cohen, Maurice, Barette, John Hine and A.M. WIlliams
"Skeleton Postmarks of England & Wales" by James Mackay, 3rd Edition 2006 compiled by Patrick G Awcock and John R Frost published by the British Postmark Society
"KRAG Machine Postmarks of Great Britain & Ireland" by Paul T Carter
"Universal Machine Postmarks" by Paul T Carter, published through the British Postmark Society
"In-Depth Slogan Postmarks Listings" by Colin G Peachey, and by Martin Grier, made available through the British Postmark Society webpage
Very great thanks to all the authors and publishers for all of the above !