July 1 (1850) to Massachusetts - Very fine strike of the large diameter sans-serif font CDS with integral 40 rate, representing the unpaid transcontinental 40 cent single rate. Manuscript California. Carried by the PMSS California which departed July 1, 1850. A typical use of this CDS.

The largest diameter SFPO CDS which included an integral rate as part of the device began service in April 1850. This is a unique style large CDS with sans-serif font and is easily identified. This CDS had three different rate versions in the device, specifically rates of 40, 80 and 6 were used.  The "6" version was created for new rate period which began July 1 1851 (the CDS was repurposed).  Some of the unique identifying characteristics of this CDS are; 
35 mm diameter (the largest size)
Sans-serif font
"Cal" is not used (rate value is in this location compared to 1849 CDS)
Day/Month only, no year
There is a period after FRANCISCO
Integral rate value (40, 80 and 6)

Examples for each version of this CDS are shown below.

May 1 (1850) San Francisco to Massachusetts - CDS with integral 80 rate, representing the unpaid  double transcontinental 40 cent rate.

July 1 (1851) to Pennsylvania - CDS with integral large 6 and straightline PAID marking, representing the prepaid  6 cent transcontinental rate which began the day this cover was postmarked, July 1, 1851.  While there is  no year date on this cover, it was used in 1851 as it is after the termination of the 40 cent rate period on June 30, 1851, and this version of the CDS stopped being used by early 1852.  This is a first day cover for this new rate.

September 17 (1850) to Massachusetts - black version of the single rate CDS.  Manuscript per Steamer Isthmus and scarce red straightline “PER STMr ISTHMUS” on unpaid 40 cent transcontinental rate cover.  Sent on the Law's Line non-contract steamer Isthmus which departed SF on September 17, 1850, its last/final trip carrying the SFPO mails. Although the PMSC had an exclusive government contract to carry the mails, George Law offered the SF postmaster the use of his ships to convey mail.  Due in part to the demands of the public, on May 14, 1850 the postmaster agreed and authorized the line to carry mail only if the letter was specifically endorsed to a Law’s Line steamer. Straightline marking devices with the vessel name were created and put into service in June 1850.  This was the last usage of the this straightline marking device. Post Office Department directive of October 10, 1850 banned all future non-contract sailings with the mails as this was a breech on the government contract.  October 19, 1850 was the final non-contract sailing date carrying the mails from the SFPO.

Earliest/Latest Found Dates (Bookends) For This CDS
Ref # Rate Date Destination Image
SF LgIntRate-1 (40)
40 April 1, 1850 Massachusetts
SF LgIntRate-2 (40)
40 May 31, 1851  Massachusetts
SF LgIntRate-3 (80)
80 April 20, 1850 NewYork
SF LgIntRate-4 (80)
80 June 1, 1851 New Jersey
SF LgIntRate-5 (6)
6 July 1, 1851 (first day of new rate) Pennsylvania
SF IntRate-6 (6)
6 March 20, 1852 Vermont