Hello while I am a newbie cat sailor. I do own an 18sq I think you are right in your assuming that you have a chopped down 18sq.

Mine was pretty much stock when I got her. Rectangular boom attached to bottom of 30' mast. Three pulleys attached inline on boom. Two torsion tubes running ontop of trampoline on either side.

I am curious if your booklets sail works though?

Also will you be adding a jib as the 18sq were unI'm because I would like to add one.

If you have any questions about the 18sq feel free to ask me.