Compared to the, earlier, Sun-Bow YA100, this Sun-Bow ELF400 was a masterpiece of style and function. Compared to a SunTour Honor, or indeed any other low-end SunTour or Shimano model of the time it was an ugly and poorly finished lump.
I have guessed its date as being 1979, only because it looks to be later than the 1977 Sun-Bow YA100, but not that much later.
I have not yet managed to make even a decent guess at what 'ELF' might stand for. I am assuming that it is an acronym and not a reference to one of Santa's little helpers.