Overbored, you win the super awesome person award for today! Your ID of the hardware seems to be spot on. Now that I've seen the Tylaska halyard shackles, I think I have those, too, on the halyards. These devices on the lifeline are slightly different, but definitely the same principle. "Knurled fid" is a great description for the opening device. The one on board is plain stainless with a spiral knurling on the "handle" end, and a hole for the lanyard. Now that I know what I'm looking for, I'm going to go order one or two. They're going to come in handy for the halyard shackles, too.
For those who are curious, I'll definitely take some better photos when we're at the boat tomorrow. I'll be able to get the brand information and such off of them, too.
To address the somewhat lively discussion of the lifelines on this boat:
The previous owner (who is also the builder, along with his wife) made the lifeline stanchions himself. They're stainless tubing drilled for the middle line and a cap fitted and mechanically bonded for the top line. The tubes fit into stanchion plates that he also designed, and then the tubes are welded to bond them to the plates.
The lifelines terminate at hardpoints on the pulpit and pushpit. There are no intermediate hardpoints. There are four openings that he (and I) call "gates," because that is how they were intended to function. These gates occur in between sets of stanchions near the bow and stern on port and starboard. The way they are set up, when the gate is open, the lifelines do lose a bit of tension, but they do not come undone. If one were to fall against them with one or both gates open on either side, they are still rigged mostly taut, held in place by swaged fittings. Closing the gates puts full tension on the lifelines. The lifeline system has no turnbuckles. Lashings are used instead to fasten the ends to the hardpoints and tension the lines.
Though maybe not today's standard, the system seems to be well designed with thought behind it. There is virtually no chance for a gate to open on its own, or to have a "false closed" position. Certainly, the lines can be stepped over if the gates cannot be opened.
Thanks for all of the responses. I was hoping someone here would know what was what, and I am not disappointed!