There is a Lang Farquharson pictured in Winfer's book but no information about how many they may have retailed.
The one pictured in the book is described as showing "typical Webley finishing work" which I take as a nod to suppose that Webley made the guns entirely and Lang had little in the making other than retailing - very common practice actually.
Of course, Lang also retailed at least one Webley 1897 and I would think that they also retailed Webley 1902 actioned rifles as well.
Mentioning this simply because the word Farquharson is used a little loosely to describe many different falling block rifles.