The Hound of the Baskervilles - the libretto?

Found - an interesting Sherlockian letter in a collection of papers of the late 'Evoe' - the comic writer, satirist, poet  and one time editor of Punch known as E.V. Knox. It is from James Edward Holroyd* dated 23/4/1967 from 11 Heath Royal, Putney to Knox in Frognal, Hampstead. He brings attention to his (Holroyd's) letter published in that weeks New Statesman and recalls a Press Club Sherlock Holmes dinner when he had sat with 'Evoe' and Sir Sidney Roberts, another follower of what Sherlockians call 'the higher criticism.' He invites him to visit old Fleet Street haunts with him ('a glass of wine at El Vino'). The published  letter reads:

Your readers may wish to be reminded of the enchanting theory, advanced by E.V. Knox, that Conan Doyle's famous story The Hound of the Baskervilles was originally written as a libretto. In support of his claim, he quoted the following stanza :

I stooped, panting, and pressed my pistol
To the dreadful, shimmering head,
But it was useless to press the trigger,
The giant hound was dead.

No wonder that on another occasion Holmes remarked : "Cut out the poetry, Watson".

Evoe's original piece A Ramble in Dartmoor published in Punch 21/1/1948 also quotes these lines of 'found poetry' from The Hound of the Baskervilles:

The night was clear and fine above us
The stars shone cold and bright,
While a half moon bathed the whole scene
In a  soft uncertain light..

He concludes 'I can only hope that we may one-day discover the manuscript of the original poem, ballad, or libretto from which the story has been reduced down into workaday prose.'

*Editor of Seventeen Steps to 221B: A Sherlockian Collection by English Writers and Baker Street By-Ways.