Many moons ago I replaced those on my 2001 Triton. I sourced 1/4" stainless steel coach bolts and re-used the white nylon washers after cleaning them up. I seem to recall using silicon in the hole.

Colin

Just found what I posted somewhere else a few years ago:-
"They are a domed or raised head coach bolt. I replaced mine with M6 stainless steel, so I guess you would be looking at a 1/4" thread. They should have a nylon washer under the head. I seem to recall giving my old washers a good bleaching and refitting them. Mine came out easily, and I replaced one at a time so I did not have any problem with bits flying about. If stubborn I would give a light tap and then apply a penetrating oil spray and be very patient. Although not ideal as this could stain an old roof I do not think you need to worry, as the old rusted bolts will probably have done this already. Obviously give a good clean with cream cleaner and a smear of mastic when putting the new bolts in."