I haven't seen that particular dropout before, but Simplex made a similar one. The hole was to hold a threaded plastic insert through which the adjuster bolt was inserted. If there's a similar hole on the non-drive side dropout, that's likely its purpose.

I'm familiar with the Simplex dropouts. Peugeot used them in the 80s. I have one on my 1982 Peugeot PXN 10. It's a good drop out that works with campy derailleurs as well. The dropout on the Sekine is different. It is not threaded. I think the purpose is just to provide a stop so you can center the wheel. First time I've seen one like this that was manufactured as part of the drop out.

The Simplex dropout isn't threaded, either; that's the purpose of the threaded plastic insert. Is there a similar hole on the non-drive side dropout?

Right, it has a piece that goes through the hole and the plastic piece threads onto it. I haven't thought about how it works since I first set it up.

No hole on the non drive side on this bike. It's just a regular drop out. So DS stop functions much like the piece that the OP linked to in the very first post. I've never seen a drop out like this before. Nice quality bike though as the main triangle is double butted chrome moly.