The upper-end Fujis back in the day were double-nutted, which made perfect sense to me, and then I didn't see any need for the spring. Unless I want a totally period-correct build, I've been using stainless hardware, with a 2.5mm-socket screw, a regular old nut, and a domed crown nut at the end. I'd rather have a milled ring i/o nut against the dropout, like the Fujis had, which are much easier to adjust by hand. But couldn't find a modern supplier. Probably just couldn't figure out the right search term for McMaster-Carr.
The stainless 2.5mm-socket screws are mega-handy not only because they won't rust, but also because driving them into the dropout with a hex wrench means my screwdriver won't be constantly slipping off the end of the screw like it does with regular slotted screws. You can also get them in several different lengths, and tailor them to any particular bike. I don't like too much screw sticking out the back, so I usually use the shortest screw that will give me the adjusting range I want for a particular dropout/frame.
Don't have any inside-slot photos handy, but on the outside of the drop they look like this:
I just run a socket head from the outside with that nut. Who needs a head on the inside? All it is doing is providing a guide to make aligning the wheel before closing the QR easier.
That said, I have two bikes with horizontal dropouts and the screws. I'd have to look to see what's on them now, 4 bikes with horizontal dropouts but two are fix gears with cogs that change regularly so no screws used, My city fix gear has screws that I've tweaked once in the past 5 years, (Chain "stretch".) Makes for easier wheel insertion after flat repairs in the dark and rain.
All those screws need is something to keep them from moving.. And that screw's only function is simply be there when you insert the wheel. Springs, nuts, thick grease, rust - they all work. (Rust can be a pain if you want to change it later so i prefer thick grease.), So, so far from rocket science.
On bikes where I remove the screws, I stuff both ends of the hole with marine grease. (Except my ti bike.)
Oh, the other reason for horizontal dropouts? Well, a vertical dropout bike is a one trick pony, It has to run with a derailleur cage (or you have to jump through some expensive hoops). Horizontal drops? Just fine with or without a derailleur cafe or tensioner. Can run an IGH or fix gear. Everything but disc brakes with no hoops to jump through at all. I spec'd horizontal dropouts on my Mooney, just so I could; knowing I never would. 38 years later I did. I"m back in love with that bike again.
One trick pony - sounds right!
Ben