Premier Hazard-Electronic Relay for Alternating Lights (Blue etc) or Horns 12/24 volt Polarity proctected Nos Ex MOD nsn 2540 99 810 7753,This is the up grade to LUCAS - H.C.3. Type Relay, Alternating Horn Control 12 volt, Lucas HC3, Two Tone Horn Relay used on Police cars , fire engines, ambulance etc