According to The 914 Restorer's Guide to Authenticity:
QUOTE
one horn was supplied as standard equipment, the second being part of the optional appearance group. A Hella 335 Hz horn was standard and mounted on the left side, while the optional Hella 400 Hz was on the right.
  • 914-635-103-11: Horn - HELLA - 335 HZ
  • 914-635-104-11: Horn - HELLA - 400 HZ

Research suggests that two horns were standard on 914-6.