I think FasterDamit is on the right tracks. As I see it from previous posts single handed the ISO fits straight into the box rules (no jib). However, if you wanted to race with a crew (two up) you would want to rig the jib but the jib is 0.5m sq to big to fit into the box rule hence the confusion. Pepin is correct it is up to the discretion of the local association chairman to decide whether or not to Grandfather a boat. Here in the UK the Stealth R has been grandfathered and that has a larger jib than the F16 rules.
I have purposely left out mentioning the Spi as I didn't want to confuse the issue but would strongly recommend fitting one if you fancy racing as an Open F16. Or just enjoy the buzz of going downwind fast but before you buy anything just make sure your mast and platform will take the extra loads.