Gray brought this cover to Muffin Break a few months ago. Here is a registered cover from Shanghai (mailed October 10, 1941) mailed to New Hampshire. It was carried possibly by an American President liner to San Pedro, California (December 27, 1941) and sent to Vancouver, Canada for censoring. Note the C. 286 censor tape used in Vancouver. The next postmarks are Seattle on February 9, 1942, then Exeter, N.H. on February 13, 1942.
After the attack on Pearl Harbour (Dec 7, 1941) some United States mail was censored in Vancouver by US censors. In December 1941, the US Censorship Branch set up a training school for reserve officers. Some officers were sent to British and Canadian censorship stations to acquire technical information and report on procedures. The US sent five officials from San Francisco and Seattle to Vancouver to assist in the censorship of considerable mail brought to California by US ships at sea in early December. US letter mail was examined in Vancouver from Dec 26, 1941, to Feb 11, 1942. Postal historians have been watching for US letters censored in Vancouver during this short period. Gray believes this is the first proven example.