It is from somewhere between July 1851 and March 1855 when the 5c rate was in force. The Sacramento City cancel type (probably would be the type 36mm across) was also in use during this period. The American Stampless Cover Catalog
would tell you this as would references specific to California postmarks.
Everything in the world is not on Google. I doubt Absalome has a Facebook page, but you never know. You need a San Francisco city directory for the years involved. The San Francisco Public Library has these online for most years.
"5" is the rate. This was sent collect/postage payable by the sender.
This is a stampless cover. "Local hand stamp" is rather meaningless; every originating postmark is "local".
A relatively short distance for sending a letter in those times...
That does not reflect the effort needed to get from Sacramento to Columbia at the time. California was hardly developed at this time. A "short" distance is not that important; do you expect (say) a businessman to leave his store to physically deliver a message to Columbia? No telegraph at this time, either. You are thinking too much in 21st century terms.
Since most letters were written to people at greater distances, such as Boston to San Francisco
Not true at all. This is hardly a unique cover. You're assuming you've seen all that there is to see(?).
Does the low rate of five cents make this cover more of a rarity ? Is it a rarity at all ?
It's not a rarity. 5c was a standard rate of the period and there are many like this to towns in the Sierra foothills. So it is uncommon but by no means rare.