When I wondered about this same question I called Hunter.
They told me to call Selden (they made my mast) because they were responsible for all hardware on the mast including the lighting. Have your boat serial number and the mast serial number handy. The mast serial number on my Selden is etched in on the port side just above the deck plate. In my case I have an Aqua Signal 40 fixture on my 2002 H410.