Mustangs destined for the RAF roll off the North American assembly line with wooden wheels, to more easily move them around the factory prior to getting their tires. If you look close, it looks like some clown wrote “do not inflate” on the wooden wheel.
Dating back to a 1930 design called the Taylor Cub, the Piper J-3 Cub design was popular as a civilian trainer and sport plane for at least three years before the US Army Air Corps ordered the first Piper L-4s in 1941 for use as a light liaison and observation aircraft. Between 1941 and 1945 the USAAF bought almost 6,000 Piper Aircraft. During World War II, Grasshoppers performed a wide variety of duties. The nickname “Grasshopper” was almost immediately applied.