Helen and Jake,
I don't know how Hall Spars seperate the two but one of the methods for building carbon tubes over a mandrel is to have a slot cut in the mandrel with an insert in the slot.
When cured it is easy to remove the insert which allows the mandrel to flex descreasing its sixe and out it comes.
There is another method using candle wax but that would not be used by Hall because it would not work because of the heat in the autoclave.
They may also cut down the sail track and then glue in a carbon tube for the sail bolt rope. This could be very neatly done and you would never tell on the finished mast as the cut was needed anyway.
This is all speculation.
It would be interesting to hear from someone who actually knows.

Regards,
Phill