This name is a synonym of Rhododendron japonicum var. pentamerum (Maxim.) Hutch..
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 50265954) with original publication details: Mém. Acad. Imp. Sci. Saint Pétersbourg, Sér. 7 16(9): 22 1870.
This name was in version 1 of The Plant List, record tro-50265954.