My morphological study of the three varieties of Malacothamnus palmeri was just recently published in Crossosoma. This assessed how different these three varieties are. The results show all three to be clearly morphologically distinct with no intermediates. It also shows them to be geographically distinct. Based on this evidence, I have raised the varieties to the rank of species as M. palmeri, M. involucratus, and M. lucianus. You can read the paper here. This includes a key to distinguish between these three taxa. A larger draft key for all Malacothamnus in Monterey and San Luis Obispo counties can be found here. ...And as photos are way more useful sometimes, see below.

Malacothamnus lucianus - Arroyo Seco or Santa Lucia bush-mallow

Note the very long stem hairs in M. lucianus, which separates it from M. palmeri and M. involucratus.

Malacothamnus palmeri - Cambria or Palmer's bush-mallow

Note the narrower stipular bracts in the inflorescence and the denser hairs on the upper leaf surface, which separate this from M. involucratus.

Malacothamnus involucratus - Carmel Valley bush-mallow

Note the wider stipular bracts in the inflorescence and the sparser hairs on the upper leaf surface, which separate this from M. palmeri. Malacothamnus involucratus from Carmel Valley have plants that appear somewhat intermediate between M. palmeri and M. involucratus but M. palmeri is not known outside San Luis Obispo County and the Carmel Valley plants still have wider bracts and sparser hairs than M. palmeri.