where find I this SF cancel and some information.
This cancel and four other double circle postmarks where use in San Francisco for 16 years, 1861 to 1876, They can be found in John H. Williams California Town Postmarks 1849-1935
Below are phone pics of the proper page showing the 5 double circle used in this time frame. The L measurement is shown by an orange line.
I'm guessing the postmark on your cover is either SAF-600 or SAF-610.
Forgot to mention the measurements are in mm.