Here is a nice cover. It has a stamp that has several types. I am leaning toward Type 1 or Type 4 or Type 5
Cover from SHASTA, Cal 1860 postmark, with receiving mark of "Pension Office" September 15, 1860. The 10 cent rate paid for the postage from Cal to WAshington DC.

Here is close up of stamp, so you can figure the type...

If any of you can figure out "pearls" "clam shells" and what is complete or not complete, please let me know!
