I have a '76 18R that was a 300ish# racer. Originally it had twins on the back for roundy rounds. Looks just like what it is a "little brother" of the 20 and a "big brother" of the 13-16s.
Mine would have to be completely rebuilt before it could be used. I rescued it from a fate of going to a landfill in Utah in 2003. Just been sitting inside ever since. Sooner or later I'll have it restored. What I like about it is it's big enough to safely hold a decent tank, 2 people, a V-6 and still run pretty serious speeds without being "over taxed" like the smaller boats would be. Could probably just as easily restore it with a GT20 like interior, hang a 115 or 150 on it and have a 4 passenger lake boat that would be pretty neat. (I'm kinda thinkin about a "tower of power", 2 seater and letting my boys play with it when they get a lil older! Guess I better get choppin on resto plans.)
Best I can tell the 18s were quite rare, just don't see or hear much about them.
RA