This name is a synonym of Rhododendron arboreum var. cinnamomeum (Wall. ex G. Don) Lindl..
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 50060261) with original publication details: Rhododendron Sikkim Himalaya pl. 6 1849.
Full publication details for this name can be found in IPNI: http://ipni.org/urn:lsid:ipni.org:names:332097-1.
This name was in version 1 of The Plant List, record tro-50060261, and has not been changed.