Your engine was made between 1938 and 1940. More precisely, if the slope back tender has a simple sheet metal "knuckle" coupler (often called the "curly-Q" coupler) which actually is compatible with Lionel postwar, your locomotive was made in 1938. If it has a long black "link" coupler, of a type associated with the early AF S gauge trains, it was made in 1939-1940. 1938 was the year A.C. Gilbert bought American Flyer and moved production from Chicago to New Haven. While the number ont he cab is often 4321, the catalog number is either 429 or 432. It will run on Lionel track ( O or O27) but the "paddle" pickups may short out on Lionel switches. It's a great prewar locomotive, some say among the very greatest of them all. It is frequently found on eBay as well as at sites that specialize in old AF such as trainmarket.com. The value depends on the condition of the locomotive and its tender and rangers from about $200 to $500+.
