Using the Jepson eFlora Key to Trifolium these plants go to Trifolium olivaceum...which (in 2010) was resurrected to species level from its previous treatment as T. albopurpureum var. olivaceum in the 1993 Jepson Manual (see note here). These plants also go to T. olivaceum in P. B. Kennedy's 1913 paper in the journal Muhlenbergia. Diagnostic characters include the plumose calyces, which exceed the corollas and were >8mm long, with tube around 2mm and lobes much longer. The comments on this species from Jepson's 1936 Vol. II of 'A Flora of California' can be read here.

This plant was shown to me in the field by Trifolium expert Randy Morgan, who commented that it:

'...almost perfectly matches the near-extinct T. olivaceum, except that it is all by itself way up Parkfield Grade rather than on the clay flats of Vacaville! Plants of the same thing with smaller heads are here & there in SCoRI but this is the only patch I've seen with head size within the range of typical T. olivaceum.'