I have examples of three different handstamps in the period 1776 to 1781, only one of which is clear enough to be a possible candidate for a new handstamp.
Firstly from June 1776, here an example of a 25½ x 6½ mm handstamp.
Secondly from December 1777 is a partial handstamp that just possibly could be an earlier example of the 30 x 7 mm handstamp without serifs known in 1781-1782.
And finally is an apparently tapered straight-line handstamp from March 1781.
It is interesting to note that a slightly tapered handstamp is known from Bristol in around this period, from 1784-1788.
In this period the British County Catalogue  has a 32 x 8 mm handstamp from 1781,with the comment that the "T" has large serifs - I don't have an example of this handstamp.