So like everyone else getting free boards to ride I ended up with one that does not have a mast base. Its the type that has a large hole in the board and what looks like a peg fits in it with rubber and then expands for a friction fit. I have been told they are impossible to find. First are they really gone gone and second can one adapt a new Chinook style one pin base to be screwed into the polystyrene board or did I just end up with a very large bookend? One note; everything else with this setup is in pristine condition! Unbelievable for a thirty year old board. Boom is undamaged, no rips in the grips, mast is near perfect and board is immaculate. I'd hate to send it down the ocean island garbage depot. This thing is never going to disintegrate.
It would be a shame to have such a nice old rig and not be able to use it so I'm sending info and advice on what might get you on the water. You're right about it never disentegrating. Those older O'Brien boards are a poly shell and will be around forever. You may have ended up with a large book end because what you have been told is true, it is very difficult to find the right type of mast base to fit. It will be an O'Brien specific design and hard to find.
You may have some luck on ebay or craigslist. One word at a time, search windsurf, windsurfer, & windsurfing. On craigslist, search the same way under sporting goods. Try any major city and anyplace near water (large rivers, lakes, coastal, etc.). You are likely to come across complete or partial rigs but you may get lucky and find someone with a spare base or willing to part with just the base.
The only other possible solution would be to possibly build one yourself. I've often thought of how a person could do it and the best I could come up with is to make a two part system that resembles the handlebar stem (or gooseneck) mounting system on older style bikes. It is where the long bolt going through the handlebar into the headset of the frame connects to a threaded part. The two ends that meet are cut at an angle. As the bolt tightens up, the angled parts push away from each other and tighten against the inside of the head. I've often thought that this design might work as a mastbase for the older boards of your type. I also think that wrapping the two pieces with a strip of rubber from an old inner tube would help give the parts grip once they are tightened.
If you are able to fashion a part to work, you will still need a u-joint and maybe an extension. Go here to find the most common modern bases and extensions around today. www.chinooksailing.com
Hope this helps.
Keep on sailing,