Per
this discussion, there are many stubby Wikipedia articles on California locales that have been created sourced to a GNIS entries. GNIS has errors and may contain locales that are not notable, per
WP:NGEO.
The plan for the cleanup is the following. For each county in California:
Create a work page as a subpage of this one, using
this template
Fill in the work page by sorting articles by notability
USPS Report on the evolution of mail delivery: it explains what a fourth-class post office was. At the height of pre-RFD service there were 76,000 of these, and yes, often enough they were just a spot in someone's house.