This name is a synonym of Penstemon angustifolius subsp. caudatus (A. Heller) D.D. Keck.
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 50079706) with original publication details: Minnesota Bot. Stud. 2(1): 34–35 1898.
Full publication details for this name can be found in IPNI: http://ipni.org/urn:lsid:ipni.org:names:187256-2.
This name was in version 1 of The Plant List, record tro-50079706.