This seems to be part of the same group from which FT879 and FT879a originate. In his manuscript catalogue Leonard Duke wrote:

D.3629. Mountain Landscape. Water colour. 7¾ x 5⅞ (sight). An almost monochrome dramatic drawing of a single high hill in background with a great shaft of light coming from behind a black cloud from left to right. I bought this drawing from the widow of my friend John Pearce Higgins, July 1960, for 10. He called it J. White Abbott, but Jim Byam Shaw of Colnaghi’s thinks, and I agree, that it is probably by Towne. It is, I think, from a group of drawings, one of which is dated 1786, which Yvonne French bought from a dealer in East Anglia as by Towne. 

In the margin Duke changes his mind, noting: “I think by Warwick Smith.”¹ Perhaps Duke had misremembered the date of 1786 and was, in fact, describing FT879a, dated 1788.