This name is a synonym of Rhododendron mekongense var. longipilosum (Cowan) Cullen.
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 12304837) with original publication details: Notes Roy. Bot. Gard. Edinburgh 19(93): 186 1936.
This name was in version 1 of The Plant List, record tro-12304837, and has not been changed.