Do love the comments from ex sperts (seems everybody use to be a Mossie sailor) that go around.
Easiest way to get the facts is to contact Phillip that sells the masts for the association (he doesn't make money out of it by the way) he does it as a volunteer.
There was 1 only batch of masts from the manafacturer which somehow had been extruded incorrectly with a undersized web in them, when Phillip became aware that they where not suitable for sloop rig Mossies without reinforcment he told buyers so (he is still using a reinforced one by the way), they are no longer being sold by the association. The current batch of masts available from the association has been correctly extruded and are suitable for all Mossies without reinforcement.
I have sailed Mossies for some 16 years now, sloop rigged, cat rigged and spinnaker rigged. Using about 6 masts, including one of the undersized web masts for cat and spinnaker for a few years (I still have it in one peice). The only mast I have ever broken was when another boat ran me over whilst capsized and the mast hit the bottom, any mast will break if put under pressure when it hits the bottom.