There is an 18sq Fleet at the Lake Norman Yacht Club near Charlotte, NC.
They have 10-15 boats. They usually only have 4-6 racing.
They used to travel to Regattas, but they quit several years ago. That is why I got rid of my 18sq.
The boats are all Nacras. They have all three different 5.5 hull constructions represented(stringer hulls, foam hulls, long tramp hulls).
Nacra made 18sqs from their standard 5.5 uni hulls, or vice-versa(their website says so).
The uni hulls have the bridle tangs on the outside of the bows.
I have never heard of a third category. The portsmouth only has 2 listed.

You could build a Category 1(light) 18sq with A Class hulls.
An F18HT could be converted to an 18sq.

This is another Class with no Class Association and unknown, missing Class rules.

It is a great sailing boat. When you are on the wire the visibility is terrific. The width makes it very resistant to flipping when hit by puffs.
I trailered mine disassembled. I could assemble it alone, and be in the water, in less than an hour.