Hi Gary,
I`ve always read that the Australian Mozzies weigh in at 85kg, I`d love it if you could tell me how you get the boats so light. The South African boats are a good deal heavier, mine weighed in at 99kg sloop, before we added spinnakers, and was the lightest boat in the fleet at the time by some 5kg.
"I had thought of wider beams and tramp etc. but the mossie has to incease in weight anyway to get to 104kg." - Again, with spinnaker kit added my boat weighed in at 107kg, I wish I had to add weight to get to 104.
I`ve come up with an estimated weight of items based on weights I`ve measured in the past, some are optimistically light for the purpose of this exercise :
Platform weight = 55kg (mine is 58kg)
mast fully fitted = 12kg
Mainsail & boom = 10kg (Mylar mainsail, battens incl.)
Rigging wires incl. halyards = 4kg
daggerboards = 4kg (for both)
rudders incl stocks = 5kg
Spinnaker kit = 7kg (incl. pole, sheets, blocks & snuffer)
Jib kit = 4kg (incl. sheets & blocks)
This totals at 101kg, and If you add mainsheets & a few other bits I might`ve forgotten, you will easily get 104kg if not more.
If you have any hints as to how to reduce weight, please let me know (send a private message if you don`t want to share your secrets with other Australians )

Cheers
Steve