This "additional stanza" was first printed in 1830.
James Sambrook: "An additional stanza, intended to follow I. lxxii, was first printed in Nicholas (1830) ii. 57, and was said to have been 'introduced, in the edition of 1746'.... Some later editors have included this stanza in their notes, and a few (most recently Hardie in 1956) have incorporated it in the main text, on the assumption that it was written by Thomson and refers to Lady Lyttelton. McKillop observes, 'It is possible that this stanza is genuine, and was intended as a companion-piece to the compliment to Lyttelton. Lady Lyttelton's death in January 1747 may have prevented publication'" Liberty, The Castle of Indolence, and other Poems (1986) 163.
One nymph there was, methought, in bloom of May,
On whom the idle Fiend glanced many a look,
In hopes to lead her down the slippery way
To taste of Pleasure's deep-deceitful brook:
No virtues yet her gentle mind forsook;
No idle whims, no vapours fill'd her brain;
But Prudence for her youthful guide she took,
And Goodness, which no earthly vice could stain,
Dwelt in her mind; she was ne proud, I ween, or vain.
[Poetical Works (1865) 2:264n.]