 DATE: Hirsch mentions that the location is "near the CHOP line",  which is boldly marked on the map at 30°W. The CHOP line, indicating  where a convoys protection CHanged OPerational control from  American or Canadian to British responsibility, was first  established on July 1, 1942, but the letter that, in his first  scene, Tyler has just received is dated May 14, 1942. Also, the CHOP  line was initially at 26°W and then 47°W, never 30°.