This name is a synonym of Didymodon rigidulus var. gracilis (Schleich. ex Hook. & Grev.) R.H. Zander.
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 35140521) with original publication details: J. Hattori Bot. Lab. 39: 519 1975.
This name was in version 1 of The Plant List, record tro-35140521.