Mason seems to have had evidence for the 1742 date sufficient to satisfy Walpole, though what that evidence was we do not know. The 'Churchyard' was, I am persuaded, posterior to West's death  at least three or four years, as you will see by my note.
The facts as to its publication, etc., may be found in Gosse's edition of "The ''Elegy Written in a Country Church-Yard'' was begun at Stoke-Poges in 1742, probably about the time of the death of Gray's uncle, Jonathan Rogers, who died there on the 21st of October.
Metrical notation: - |- |- |- |- / - |- |- |- |- / - |- |- |- |- / - |- |- |- |- /Metrical foot type: iambic (- )Metrical foot number: pentameter (5 feet)Rhyme scheme: abab Rhyme (stanza position): cross (abab)Syllable pattern: .10Stanza: quatrain (4 lines)Genre(s): heroic quatrain, elegiac stanza, graveyard school, elegy Theme(s): hopelessness, vanity of life, night, social order, rural life, death ] [Era gia l' ora, che volge 'l disio A' naviganti, e 'ntenerisce 'l cuore Lo di ch' han detto a' dolci amici addio: E che lo nuovo peregrin d' amore Punge, se ode] — squilla di lontano Che paia 'l giorno pianger, che si muore.
[For I see in my thoughts, my sweet fire, One cold tongue, and two beautiful closed eyes Will remain full of sparks after our death.] was begun at Stoke-Poges in the autumn of 1742, probably on the occasion of the funeral of Jonathan Rogers, on the 31st of October. (Price sixpence).'' There was a preface by Horace Walpole.
Walpole did not at first accept the account of the date of the poem, submitted to him by Mason before the Memoirs of Gray went to press. 1, 1773:''The 'Churchyard' was, I am persuaded, posterior to West's death  at least three or four years.
At least I am sure that I had the twelve or more first lines from himself above three years after that period, and it was long before he finished it.''And yet Mason appears to have satisfied Walpole that the opinion expressed in the Memoirs was correct, for Walpole writes to him Dec.