Collection location: Menlo Park, San Mateo Co., California, USA [Click for map]
Saw lots of these, and thought they were alnicola. They really look like it, and they can be about the same, although alnicola has more zoning on the cap, and less scrobiculation on ave.
But the one clear difference is the milk in the gills, these have a yellow staining milk, and yellow stained gills. So they are scrobiculatus.
In the last two photos it is easy to see the yellow stained gills. But the last photos is supposed to show the yellow stained milk, I cut two parallel lines in the gills there. You can almost see the line of yellow milk if you look close.