The covers, being sold by Tennants Auctioneers in North Yorkshire on October 7 include Berwick, Morpeth and Newcastle.
The highlight of the group of six covers, which are being sold with an estimate of £350-£450, is a cover bearing the very rare 1st type mileage mark for Berwick 337 – the 337 denoting the number of miles from London.
The 1st type mark for Berwick can be recognised by the black line either side of the numbering.
The new postal rates and charges of 1784 were based on the distances a letter was carried and the number of sheets the letter contained.
The mileage from London began to appear on the provincial postmarks, as the majority of letters at that time being addressed to London.
Each postmaster would have a handstamp with the name of the town and its distance from London.
By including the mileage from London within the handstamp the postmasters were able to calculate the intermediate distances of the various towns and thereby apply the postal charges that were due for each letter.
There were many inaccuracies to this system and within a few years it was discontinued and replaced by normal town handstamps.
John Carey, the renowned mapmaker, was commissioned to measure every post road in the country during the next few years, resulting in the reissue of the second type mileage marks in 1801 with many revisions to the first types.
An illustrated catalogue is available at www.tennants.co.uk leading up to the sale.