February 28 (1850) to Maine - CDS with integral 40 rate, representing the unpaid transcontinental 40 cent single rate. Manuscript pr Oregon March 1, 1850. Carried by the PMSS Oregon which departed March 1, 1850. A typical use of this CDS.

The first SFPO CDS which included an integral rate as part of the device began service in February 1850. This CDS replaced the 1849 version and also the 1849 boxed rates (as the rates were included in this new CDS).  Surviving evidence shows a "clean transition" from the 1849 CDS to this CDS (no overlap usage, the transition occurred in February 1850). This CDS had several versions due to the ability to swap-out the rate slug in the device, specifically rates of 40, 80, 12-1/2 and 2 were used. There was also a version which utilized "RECd" in the device, as well as a no-rate blank (slug removed) version. Some of the unique identifying characteristics of this CDS are; 
33 mm diameter
Serif font
"Cal" is not used (rate value is in this location compared to 1849 CDS)
Day/Month only, no year
Integral rate value (40, 80, 12-1/2, 2, "RECd" and Blank)

Examples for each version of this CDS are shown below.

April 26 (1850) San Francisco to Stockton, CA - CDS with integral 12-1/2 rate, representing the unpaid West Coast single rate. Unlike the 80 double transcontinental rate version, there is no "25" double rate version of this CDS.

July 15 (1850) to New York - CDS with integral 80 rate, representing the unpaid  double transcontinental 40 cent rate.

August 9 (1850) incoming to San Francisco - CDS with integral 2 rate, representing the local drop rate.  Manuscript pr kindness of Mr Woodman.  Letter datelined Mexico City, May 14th, and privately carried to San Francisco where it was deposited into the PO.  This CDS with integral "2" rate is somewhat scarce and surviving evidence shows it to be used only in the last half of 1850.  Letter discusses copper mining in Mexico. 

May 15 (likely 1851) to Connecticut - CDS with "blank" (no) rate and separate 40 rate handstamp, representing the unpaid transcontinental 40 cent single rate. Note the downward shift in location of Day/Month as the rate slug has been removed from the device.  This CDS style and the 40 rate marking were in use both for May 1850 and 1851, so either year is possible.  This cover is likely from 1851 as that was the sailing day for the contract steamer PMSS Northerner. The May 15, 1850 sailing date was the maiden voyage of the non-contract steamer Isthmus of George Law's Line.  Although the PMSC had an exclusive government contract to carry the mails, George Law offered the SFPO postmaster the use of his ships to convey mail.  Due in part to the demands of the public requesting more frequent mail departures, the postmaster authorized the line to carry mail only if the letter was specifically endorsed to the steamer. Since there is no endorsement of "Isthmus" on this cover it is assumed to be carried in 1851 (no endorsement was necessary for mails carried on the contract PMSC sailings).   

The above cover entered the mails at the Panama City US mail agency where it received the straightline "PANAMA" marking. Carried by the PMSC Panama to San Francisco which arrived on September 21 (1850), however the mail was delayed on the steamship and it was postmarked September 30 at the SFPO. The "RECd" marking in the CDS is rare and found on very few San Francisco incoming steamship letters. Red Box-30 due marking is for the 30 cent contract steamship rate to/from Panama (changed to 20 cents on July 1, 1851). This is the only recorded westbound "Panama" usage, Illustrated in Letters of Gold, p. 73.

Earliest/Latest Found Dates (Bookends) For This CDS
Ref # Rate Date Destination Image
SF IntRate-1 (40)
40 February 20, 1850 Maine
SF IntRate-2 (40)
40 August 31, 1850  New York
SF IntRate-3 (12-1/2) Earliest12-1/2April 26, 1850Stockton, CA
SF IntRate-4 (12-1/2) Latest12-1/2June 19, 1850San Jose, CA
SF IntRate-5 (80)
80April 1, 1850Connecticut
SF IntRate-6 (80)
80November 15, 1850Rhode Island
SF IntRate-7 (2)
2August 6, 1850San Francisco (Local Drop)
SF IntRate-8 (2)
2October 25, 1850San Francisco (Local Drop)
SF IntRate-9 (Blank)
1.60February 28, 1850Wisconsin (x4 rate, legal documents)
SF IntRate-10 (Blank)
22July 1, 1851France
SF IntRate-11 (RECd)
40April 6, 1850Incoming to SF, from Sacramento, for Massachusetts
SF IntRate-12 (RECd)
20November 5, 1851Incoming to SF, from Valparaiso.  Note position of RECd marking at top, not bottom