I feel a bit silly, because I did a long search resulting in my post above before checking Colin Cummings' "
Thought without Anger". The books list the serial of the three US aircraft: 42-100699, 42-100706 and 42-100701. It also adds that there is an unconfirmed report that the two men killed on the ground were sleeping or working on one of these aircraft and were probably USAAF.