WW2 censorship of transpacific mail at San Francisco

I have a registered cover from Calcutta dated 15 November 1941 that definitely caught the last PanAm FAM14 flight from Singapore on 29 Nov, because it was datestamped in San Francisco on 6 December. My experience suggests that registered mail passed through censorship quicker than ordinary mail, so I am now confident that most ordinary airmail posted elsewhere in India after 15 Nov would not reach Singapore in time. The last FAM19 flight from Auckland had left on 25 November, so mail by that route would have had to be posted before mid-November too.
Transpacific airmail to the USA & Canada that missed these flights was not returned westwards on the BOAC ‘horseshoe’ route. It continued eastwards and was accumulated at Sydney, where the American ship Mariposa was about to depart for San Francisco. She sailed from Sydney on 17 Dec with the accumulated mails, and arrived at SF on 30 December. A couple of covers from Australia to Canada are postmarked 3 January 1942 and a postcard from India to the USA has a filing note of 5 January, so were definitely carried on the Mariposa, but none of these items is censored.
After that, a few covers to USA posted in India on 23 & 24 Nov, and a registered cover posted on 4 December, have US postmarks between 2 and 9 February, which clearly indicate a second or third batch arrived by ship.
Covers posted between 20 November and 12 December sometimes have US censor labels, always with San Francisco numbers, but no arrival datestamps. And alongside these, there are many covers with neither labels nor datestamps.
Does anyone know what selection criteria were used for censorship at SF; and when the labels were introduced there? And does anyone know which ships brought transpacific mail to San Francisco after the Mariposa?
Transpacific airmail to the USA & Canada that missed these flights was not returned westwards on the BOAC ‘horseshoe’ route. It continued eastwards and was accumulated at Sydney, where the American ship Mariposa was about to depart for San Francisco. She sailed from Sydney on 17 Dec with the accumulated mails, and arrived at SF on 30 December. A couple of covers from Australia to Canada are postmarked 3 January 1942 and a postcard from India to the USA has a filing note of 5 January, so were definitely carried on the Mariposa, but none of these items is censored.
After that, a few covers to USA posted in India on 23 & 24 Nov, and a registered cover posted on 4 December, have US postmarks between 2 and 9 February, which clearly indicate a second or third batch arrived by ship.
Covers posted between 20 November and 12 December sometimes have US censor labels, always with San Francisco numbers, but no arrival datestamps. And alongside these, there are many covers with neither labels nor datestamps.
Does anyone know what selection criteria were used for censorship at SF; and when the labels were introduced there? And does anyone know which ships brought transpacific mail to San Francisco after the Mariposa?