I love that out of all the more complex ideas about what may have been used, it turns out likely to be a solid piece of metal with of a given thickness, couldn't get more simple than that.
If you're into 3D Printing, feeler gauges are also a great way to level your bed... move the extruder 0.04 above the bed at four points, stick your 0.04 feeler under it and adjust the springs until it touches the feeler. Voila, one perfectly leveled 3D printer bed.
that's a weird in-between two more popular ways of doing it.
The non-precise way you've already heard about, sheet of paper, check for dragging.
The better (more precise) way, is using a magnetic base micrometer attached to your extruder head. You can then watch the bed run-out in real time; if your 3d printer supports many configurable sections across the bed ( I know Prusa style cartesian units mostly all support quadrants) you can record the run out everywhere across the plane without any more physical work than watching the gauge and recording the results.
P.S. be careful using both the micrometer and the feeler gauges on a 3d printer bed. Most work plates now-a-days are using PEI coatings that'll scrape off easily with metal-on-metal contact.
I like the sheet of paper method, but I have gotten a lot more use out of a $0.15 feeler gauge and I get consistently better prints... but I don't have the tools to scientifically quantify it so it could be placebo.
They're deceptively simple. If you want to get into the history of modern manufacturing, a lot can be written about the development of gauges of accurately known dimensions.
Go find a PDF of "Fundamentals of Mechanical Accuracy" by Wayne R. Moore, if you haven't already seen it. That book, along with George Daniels text "Watchmaking" are some of the top inspiring works on the mechanical arts in my collection.
Somewhat sadly a lot of the drive for that tolerance was weapons, until we could accurately make very high tolerance (for the day) parts you couldn't use parts from one rifle on another without a smith, it certainly wasn't field serviceable - this imposed a lot of maintenance overhead until they solved it properly.
It seems like we really like to push the limits when it comes to shooting each other.
That the tolerances are so precise is exactly why you can do that on most modern rifles. It depends on the design - specifically, how they achieve headspace. On an AR-15, or almost any firearm with a similar bolt locking arrangement, bolts - indeed, entire BCGs - are swappable in practice, and people do that routinely without bothering with gauges regardless of what the manual says. On AK and similar designs, yeah, that's a really bad idea.
> A complex system that works is invariably found to have evolved from a simple system that worked. A complex system designed from scratch never works and cannot be patched up to make it work. You have to start over, beginning with a working simple system. - John Gall
Somehow your comment reminds me of a pissing match I saw between a young aerospace engineer and an old retired one. The old guy was right in his argument that it doesn't matter what the constraints on materials and dimensions are, it that they come from somewhere and are documented.
His example was designing brackets for a Indonesian construction company that was using locally sourced tropical softwoods for constructing buildings. The brackets were designed to be made with 'steel' and the tolerances were compatible with a hungover guy running a drill press.