Never heared about Löwenherz fasteners, and I got a BMW from 1928 with Bosch Magdyno. Certainly they had some M 4,5mm and 5,5 mm screws, M 3.5mm electrical norm screws, but these are common 60 degrees profile. BMW at that time had M 6x0.85 screws , a pitch that nobody else used. Well, that was long before some standardisation seemed a necessity. Not so rare are M 7 x1 screws and bolts, used on Horexes, Citroen, Russian aircraft, and lots other types , still easy to identify without mysterious files to look up .
We don´t need to speak about special segments of trade like round thread electric household bulb sockets, yes, Deckel mills had collets with S20x2 mm saw tooth thread profile, not to mention a myriad of weight and length names.
Another weird way of defining head hexagons is wanting a spanner for a BSW bolt 5/8 or so, you cannot tell from that type what size the hex head is across flats. Got a different screw with unknown thread, so what spanner will you hunt in your garage for this case ?? So get out a vernier and take the size. Then look up a file to compare that size to any norm that happens to have same spanner ? No wonder anybody in imperial world is seen carrying a monkey wrench around 24 hours a day , hopeless to know what he is going to need for next bolt. A real mess , even without having metric fasteners in your garage. My suspicion is tradesmen had in their mind a wish to discourage Joe Public from tackling any job the tradesmen wanted for themselves and to show off their "impressive trade skills" . Or why would one want to grade wire sizes, drills, small screws, electric wires in numbers and not by real measurable sizes in inch or millimeters for checking what you got before you ? And that with smaller size at higher numbers - what sick brain has come up with this logic ?? It is just right that only 5 percent of world population still remain in this obsolete age . . . . .
Vic