This name is a synonym of Rhododendron agastum var. pennivenium (Balf. f. & Forrest) T.L. Ming.
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 12304718) with original publication details: Notes Roy. Bot. Gard. Edinburgh 13(61): 55 1920.
Full publication details for this name can be found in IPNI: http://ipni.org/urn:lsid:ipni.org:names:333059-1.
This name was in version 1 of The Plant List, record tro-12304718, and has not been changed.