The only thought that I can suggest is to take the boat off the trailer, either roll her over enough to extend the CB or lift her up on saw horses. Then measure as best you can where the pivot bolt is, then transfer the measurements to inside the cockpit. I'm familiar (sort-of) with the old 15, but not "intimately" familiar..... so, based on the DS II (which I am familiar with) I'd say that close-enough should be close enough....and so, locating the inspection ports on the cockpit sole just aft of the forward end of the CB trunk should work. On the DS II and Javelin the ports are about as far forward as possible on hte cockpit sole. I have to reach slightly forward into the bilge to reach the pivot bolt (About 6-8"). I checked Today when I re-installed my CB. I've added 2 pics of a 1970 O'DAY 15 that has inspection ports, I'm surprised that your boat didn't come with them, but OH well! Those pics should give you a rough idea of where to install the ports.