It was never common knowledge to Americans, either -- except for a few Americans in the vehicle lighting industry. :)
1940: the first standardised sealed-beam headlamp, a 7-inch round unit, 6 volts, 40/30 watts (high/low), called the 4030.
1950 or so: wattage is increased to 50/40 (high/low) and the new lamp is called the 5040.
1956: aiming pads (three little protrusions in a triangular formation on the front of the perimeter of the lens) and a filament shield (to block stray light that causes backdazzle in bad weather) are added. Wattage stays the same at 50/40. 6v unit still called 5040, new 12v unit called 5400
1958: The reflector focus is improved. Wattage stays the same at 50/40. There is now an effort at a standard system of nomenclature: the first digit indicates the size, the next digit indicates the service application (0 for headlamp), the next digit indicates the revision/version, and the final digit specifies the subtype (high/low beam headlamps, known as "type 2", have a 2 as the final digit; high-beam "type 1" headlamps have a 1).
For whatever reason, the first digit for 7-inch headlamps is "6", while the first digit for the new 5.75-inch headlamp is "4". (the other major market where sealed beam headlamps are used, the British Commonwealth, uses "7" for the first digit of 7" lamps and "5" for the 5.75" lamps). So in the American system, the new 5.75" lamps are 4002 for the 37.5/50w high/low beam (4005 for the heavy-duty version), and 4001 for the 37.5w high beam. The 6v 7-inch unit is now called 6006, and the 12-volt unit is called 6012.
This system of nomenclature breaks down very quickly as the industry seems to make and break rules at random:
1970: the 7" headlamp is upgraded to 60/50w and is now designated 6014. Heavy-duty or long-life versions by various makers are 6015 or 6016. The 5.75" high/low beam headlamp is upgraded to 37.5/60w and is now designated 4000. A heavy-duty, vibration-resistant/long-life version of the 4000 is called the 4040.
1975: the small rectangular (165 × 100mm) four-headlamp system comes along. The 40/60w high/low beam is 4652 and the 50w high beam is 4651 ("46" makes sense because those metric dimensions are more or less 4" × 6", but now we're using the first two digits for the size…)
1977: the big rectangular (200 × 142mm) two-headlamp system comes along. The 65/55w lamp is called 6052 (now we're using "6" for something other than a 7" round lamp, and a random "5" to differentiate from the "1" in 6012, but even though we've got about the same filament wattage as the 6014 lamp, we're using "2" as the final digit…).
1977 or so: GE buys a couple million 60/55w H4 halogen bulb burners (just the glass capsule with filaments inside) from two European makers and begins making halogen sealed beams: H6014 7" round and H6052 200 × 142mm rectangular. Wagner follows suit around 1980, only they call their rectangular unit H6050 (just whatever, pick any digit; they're all good).
1979: US regs change to allow double the high beam intensity. New 5.75" round high beam is a 50w item called 5001. Halogen version is H5001. Halogen small rectangular 50w high beam is H4651.
1984: US auto industry wants lower-wattage headlamps so they can use cheaper switches and wiring. US lighting industry devises a homegrown 35w halogen capsule. Poorer-but-still-legal performance. Now we've got the H6024 (7-inch round halogen 35w high/low beam…except Philips calls theirs H6017), the H5006 (5.75" round 35w high/low beam), the H4656 (small rectangular 35w high/low beam), and the H6054 (large rectangular 35w high/low beam).
1987: Truck-duty extra-long-life halogen sealed beams: H5024 7" round, H5054 large rectangular, H5062 and H5051 small rectangular, and H5501 and H5506 (or H5009) small round—now we've decided the first digit doesn't mean anything and the rest of them are pretty much random.
1992: higher-performance sealed beams with what used to be the standard wattages (65/55 or 60/55 high/low), but we keep the designations adopted for the low-wattage lamps and just add whatever suffix each individual maker likes. GE calls their "High Output" lamps H6024HO, H6054HO, and H4656HO. Sylvania calls theirs H6024XV, H6054XV, and H4656XV. Wagner calls theirs H6024BL, H6054BL, and H4656BL.
So now there's pretty much nothing left resembling any kind of standard numbering system.
Notes:
1. Years are approximate in this longer answer than you probably wanted, and it doesn't go into some of the little-known but really cool sealed beams made in places like Japan, the UK, and the Netherlands.
2. I sometimes wonder what it's like to think of car lights in terms no more complicated than present/absent, works/doesn't, on/off, bright/dim, and white/red/amber.
