The "Tractor Expedition" marking (also known as the Tractor Party marking) from the 1924 British Mt. Everest expedition (EV24-M4) was used exclusivly by the Tractor expedition. All covers bearing the marking were mailed through the Yatung, Tibet postal collection facility. It's interesting to note that even though the expedition did not reach Tibet, the mail was still forwarded to the Yatung facility before being routed back through Siliguri, India. The earliest known use (EKU) is on the two covers on June 5, 1924 from Yatung, Tibet (EV24-M4_006 and EV24-M4_011). Even though the Tractor expedition started after the main climbing expedition, it was the first to use any of the expedition markings. June 5th is the EKU for any of the 1924 Everest markings.