The BISHOPMARK was the first type of postmark introduced in Britain by HENRY BISHOP (who was Postmaster General between 1660-63) as a datestamp in two round segments which could be changed daily and was applied to Letters in LONDON so that they were accurately dated and helped to make sure that Letter Carriers were not delaying mails.  Earliest dated is from 1661 are the first few years are very rare with 1661 especially sought after together with London Plague and Great Fire years of 1665 and 1666 not far behind.  They become more plentiful from the later years of the 17th century and into the 18th century so collectors should try to get a good clear example on a complete Cover or Entire Letter. The earliest examples are 13mm in diameter and this was gradually increased into the 18th century which can be found from 14mm to latest examples in the 1780s which are around 20mm diameter.  Originally the MONTH was in the TOP segment and the DAY in the LOWER segement but this was reversed in 1713 to become DAY ABOVE MONTH and this is handy to remember if you are ever buying a really old Letter which may otherwise be un-dated.  In 1787 an EXPERIMENTAL BISHOPMARK came into use with the MONTH AROUND THE CIRCLE and these are RARE generally as this version only lasted less than 4 months when the more common later LONDON circular datestamps (cds) were introduced and these make a neat little series to collect.DAVID SHAW'S OLD LETTERS