Two different things I think
I think we are talking about two different fittings. On my '79 Oday 23, there are two stand up blocks (one on either side) that are mounted to the side decks. These are for the jib sheets. A way further aft are two small u-bolt looking fittings, also one per side. I don't remember exactly how far back they are mounted, but your suggestion that they are alternate lifeline termination points makes sense. The stern pushpit (where my lifelines terminate) was an option. If one had ordered lifelines but no pushpit, well, you would still need a place to terminate them. I hadn't thought of that as a use for the things before. I thought they were alternate places to affix the main sheet when on a run. When the boom is way out there and the main is attached to that triangle on the backstay, the sheet itself is really in the way. I use a snapshackle on the lower main block, and its easy to remove it and put it on one of those u-bolts.