Hi Julie,
Here is what I have on Boonville postmarks. For the time period and the life time of the stamp the postmark should be Men-140, of which there has been one date reported. Real shame the postmark isn't more visible, but as you'll probably remember, I'm always interested in a Mendocino County cover.
This post office was one of the first post offices in the county, opened as Anderson on 18 Oct 1858, the second office opened in the county, but there are no Anderson postmarks recorded, though I have seen two manuscript covers sold on
ebay in the last 20+ years.
The heading
PM COMP is taken from Alan Patera's booklet California Postmaster Compensation which tells what the postmaster was paid for the year. an indication of the size of the office.

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