I have a Prindle "Escape" which is the 16 footer's rig, on the 18' hulls, and it is boomless.

Look at the aft, bottom edge (the clew) of the mainsail, see if there is a metal plate about 2" wide by 18" long, with about 6 holes in it.

If there is, you have a boomless rig, so you simply shackle the blocks directly to one of the holes in the clew plate. I didn't like the hassel of using a shackle to attach the blocks so I bought a stainless steel 4" carabiner at Home Depot for about $5, put the top main blocks to it, then clip it into the clew holes. It works great and I never have to unscrew the shackle when I'm done sailing.