Please post a close-up pic of the number, as so far the first digit doesn't make sense. It could be either a 0, 1, 2, or 3 and of those I'll guess it is either 0458 (Tues. Feb. 14, 1978) or 3458 (Mon. Dec. 11, 1978). Either way that makes it a '78 model. For reference here is a pic of a '76 Superior badge stamped 2106, which means the bike was built on Wed. July 28, 1976: