August 1 (1849) to Massachusetts - first day usage of the first CDS employed by the SFPO  (and the last date of usage of the SF straightline postmark), and also first day usage of the PAID straightline marking.  Additional manuscript Paid 40 in magenta ink, representing the paid transcontinental 40 cent single rate.  Cover with gold miner's content; people are coming every day by the hundreds; we shall sail up the river Monday, they say it is hard work, but a man can average about two ounces per day

The first SFPO CDS debuted on August 1, 1849 (this was also the last usage of the straightline). This CDS is similar in style to others used by the SFPO in later years. Some of the unique identifying characteristics of this CDS are;
33 mm diameter
Serif font
Lower case "al" in "Cal." with period
5 mm gap between "S" of SAN and "C" of Cal (a large gap compared to other devices)
Month/Day only, no year (typically marked the 1st of the month, per the planned steamer sailings)
Separate rate marking

This was the only CDS used in 1849 (and continued into 1850). Covers with this CDS are known with additional manuscript rate, "Paid" and "Ship" markings (magenta, black and red ink).  An auxiliary straightline PAID device also began being used on August 1, 1849 (see the above cover) and can be found on prepaid covers.

In addition to this CDS and the straightline PAID marking, the B0x-2, Box-40 and Box-80 rate markings appeared in 1849 (see below).

September 1 (1849) to Massachusetts - manuscript 80, representing the unpaid transcontinental double 40 cent single rate.  The second month use of this CDS which debuted August 1, 1849.

The Boxed-Rate devices arrived at the SFPO in September 1849 and the 40 & 80 rates began being used for the steamer departure on October 1, 1849 (PMSS Oregon).  Above depicts the begining of the transition from manuscript rate markings to the Box-40 (and 80, not shown) in late September 1849.  The  September 27 CDS used to Sacramento City shows a mistaken application of the Box-40 rate which was then crossed out and downrated in manuscript to the correct 12-1/2c West Coast rate.  This cover confirms the devices were in existence prior to the October 1 sailing date (but after the September 1 steamer departure as covers with that date only contain manuscript rates).  The bottom-right cover is a first day usage of the new Box-40 device.  Boxed rate markings were used for the remainder of 1849 (up to February 1850 when a new integral rate CDS design replaced them) as the manuscript markings stopped being used. 

The example above shows the Box-2 marking used for the local SF drop rate (the CDS is not used for in this application).  This cover (Ex-Haas) has pencil notations stating the letter was dated September 18, 1849, although the letter is no longer enclosed.  The Box-2 earliest found use (thus far) is this September 18 (1849) cover.  The 2, 40 and 80 Box-Rate devices appear to have been delivered as a set to the SFPO and began service in the second half of Setember 1849.

Earliest/Latest Found Dates (Bookends) For This CDS
Ref # Rate Date Destination Image
SF 49CDS-1 40 August 1 1849 Massachusetts
SF 49CDS-2 40 February 4 1850  Maine