This name is a synonym of Penstemon rydbergii var. aggregatus (Pennell) N.H. Holmgren.
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 50079702) with original publication details: Amer. Midl. Naturalist 33(1): 158 1945.
Full publication details for this name can be found in IPNI: http://ipni.org/urn:lsid:ipni.org:names:187751-2.
This name was in version 1 of The Plant List, record tro-50079702.