This is the feature called Mitchell Rock on the Mount Diablo State Park map and on the signpost in the park. It's just a rock above the namesake Mitchell Rock trail, with almost no prominence. See the picture attached to the Mitchell Rock page as it is currently in peakbagger.com, from Clint Baechle's trip report. The signpost seen in the picture clearly shows the altitude of 1080 feet. The feature named Mitchell Rock on peakbagger.com and on some maps is shown as unnamed point 1507 feet on the Mount Diablo State park map.