This name is a synonym of Mila caespitosa Britton & Rose.
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 5101351) with original publication details: Descr. Cact. Nov. 11 1957.
Full publication details for this name can be found in IPNI: http://ipni.org/urn:lsid:ipni.org:names:135674-1.
This name was in version 1 of The Plant List, record tro-5101351, and has not been changed.