So far BugGuide has moved it from ID Request into subfamily Eumeninae, no finer ID yet.
Clicking through 281 pages of images, I found a couple of
close visual matches that were labeled "Perhaps close to
Ancistrocerus tuberculocephalus?"
To my eye most of the images of that species are not a good match, but
one image of the earlier specimen (from 5100 ft elevation in California) has been confirmed as
Ancistrocerus tuberculocephalus. The fellow who did that ID, Richard Vernier, seems no longer active on BugGuide (last comment Feb 2015).
http://bugeric.blogspot.com/2011/07/was ... cerus.html has some discussion that seems to match the circumstances well: "
Ancistrocerus tuberculocephalus is known to use the abandoned mud nests of the Black and Yellow Mud Dauber,
Sceliphron caementarium, for its own nests. "
--Rik
Update: 10 April 2022, my post at
https://bugguide.net/node/view/1520275#3429339 was updated by bdagley to be genus
Ancistrocerus.
Update: 14 August 2022, update by bdagley to be
Ancistrocerus tuberculocephalus.