http://www.paeo.de/h1/hand_maz/pioneer/04biography.html
published
1 box of correspondence between Heinrich Handel-Mazzetti and William Wright Smith dating from 1922-1930.
There is also correspondence between Dr. H. Handel-Mazzetti and W.W. Smith filed under "Smith, Sir Wm. W.", "Rhododendron Corresp." folder Handel-Mazzetti - copies of these letters are also in the box.
Collection is open to researchers by appointment, see (right click, open link in new tab:) https://www.rbge.org.uk/science-and-conservation/library-and-archives/visiting-the-library/
Permission required from RBGE