Hi
That looks to be conclusive proof of a connection to the Masons of 'The Artillery Ground'.
Mason is a relatively common surname so there will be lots of them in London, but as a starting point have you eliminated this PCC will
Will of William Mason of Saint Mary Whitechapel , Middlesex 19 March 1782 PROB 11/1088
since the death falls between the 1774 marriage and the 1795 will of Rebecca and the Mason family had a tendency to leave PCC wills and William of the other two brothers was known to be married?
Have you also knowledge of these documents held by Somerset Record Office.
MASON FAMILY deeds.wills.etc. DD\BR\pp/1 1760-1857
Deeds relating to the Mason family of Chichester and London, etc
1 bdle.
Contents:
Probate wills of Rebecca Coyde of Broughton Gifford, Wilts., wid., 1831, with accounts re.probate and burial certificate; Timothy Jordan of Edmonton, Middx., 1785, Thomas Mason, of Old Artillery Ground, Liberty of the Tower, London, weaver, 1760; Thomas Mason of Southampton, gent., 1802; John Plaisto of Chichester, Sussex, wine merchant, 1823; Peter Pousett of Greenwich, gent., 1810.
Marriage settlement of John Plaisto of Tower Hill, wine cooper, and Susanna Mason of Tottenham, Middx., 1775, and correspondence re. estate of Rebecca Walford neé Plaisto, 1857. Assignment of legacies to Thomas Mason of Chichester, 1813-1817. Assignment of legacy under will of Thomas Mason (1802) by assignees of Luke Evill, bankrupt, to James Hooper, of Bath, hatter.
MASON FAMILY deeds DD\BR\pp/2 1810-1819
MASON FAMILY deeds, etc. DD\BR\pp/3 1714-1817
MONCK FAMILY deeds DD\BR\pp/4 1809-1824
PORTSEA (co.Hants.) DD\BR\pp/5 1782-1812
Regards
Valda