Carpenter's Reference Guide, p. 87, offers the following interpretations of the initials: William Lawson, William Lee, William Leigh, William Leighton, William Lewin, William Lawes, William Lombard, and ('the best guess') William Lyon.
Commendatory Verse from The Faerie Queene (1590), p. 599 (repr. in all editions):
When stout Achilles heard of Helens rape
And what reuenge the States of Greece deuisd:
Thinking by sleight the fatall warres to scape,
In womans weedes him selfe he then disguisde:
But this deuise Vlysses soone did spy,
And brought him forth, the chaunce of warre to try.
When Spencer saw the fame was spredd so large,
Through Faery land of their renowned Queene:
Loth that his Muse should take so great a charge,
As in such haughty matter to be seene,
To seeme a shepeheard then he made his choice,
But Sydney heard him sing, and knew his voice.
And as Vlysses brought faire Thetis sonne
From his retyred life to menage armes:
So Spencer was by Sidneys speaches wonne,
To blaze her fame not fearing future harmes:
For well he knew, his Muse would soone be tyred
In her high praise, that all the world admired,
Yet as Achilles in those warlike frayes,
Did win the palme from all the Grecian Peeres: