Carpenter's Reference Guide, p. 87, proposes the following explanations of the initials: Robert Sackville, Sir Richard Smyth, Robert Sidney, Richard Stapleton. But a letter by C.M. Millican to the TLS, 7 August 1937, p. 576, suggests Robert Salter, chaplain to Lord Sheffield, as the most likely author of the lines. See also No. 69.
Commendatory Verse from The Faerie Queene (1590), p. 598 (repr. in all editions) :
Fayre Thamis streame, that from Ludds stately towne,
Runst paying tribute to the Ocean seas,
Let all thy Nymphes and Syrens of renowne
Be silent, whyle this Bryttane Orpheus playes:
Nere thy sweet bankes, there liues that sacred crowne,
Whose hand strowes Palme and neuer-dying bayes,
Let all at once, with thy soft murmuring sowne
Present her with this worthy Poets prayes.
For he hath taught hye drifts in shepeherdes weedes,
And deepe conceites now singes in Faeries deedes.