Skipping over the period from 1800 to 1818 (as I don't have any of the four handstamps from that period), one comes to a series of circular handstamps of different sizes with the mileage to London (161). Firstly one with 'o's in the handstamp from 1817.
Then a couple of examples of a slightly smaller handstamp without any 'o's. The letters in "TAUNTON" are more narrowly spaced than a later handstamp.
Then another example of the narrow spaced handstamp above and an example of a wider spaced one from 1828.
And finally a further example of the wider spaced handstamp, also with an undated Taunton Penny Post handstamp.