DOT and Stupid Lighting Rules:
In the beginning, there was darkness, and this was bad. It tended to make night driving rather difficult; not being able to see the road, or the signs, or the tree you're about to hit tends to dampen the spirits of even the most adventurous driver. So, the automotive headlamp was invented to dispel the darkness.
Initially, many different solutions were tried. Gas lamps, acetylene lamps, phosphoric coatings, lanterns, candles, blazing torches (a Transylvanian regional peculiarity - most often teamed with pitchforks), and other sources of illumination were tried. Eventually, manufacturers settled on an incandescent electric bulb (mounted in a parabolic reflector with a transparent lens) as the best solution. Unlike some of the other alternatives, it did not need to be separately refueled, was easy to replace when it was no longer servicable, and (perhaps most importantly) did not explode or tend to set the carrier vehicle on fire.
In these early days, there were no standards - truly, most bulbs were unique to the maker or even the individual car. Beam patterns, light intensity/output, even the size of the lens was up to the manufacturer's discretion. It didn't seem to matter too much, because the lighting systems of this period were rather inefficient, low output, and marginal at the best of times. As long as you could see the road, nobody cared. This state of affairs continued from before the Model T until just after the Second World War, though there was some effort at standardization during this period.
After the Second World War, the US maintained an armed presence in Europe as a deterrent to Soviet aggression, thus starting the Cold War. This meant that large quantities of young men got shipped off to Europe, where car design philosophies were completely different than they were at home. Naturally, these young men would purchase vehicles where they were stationed (for various reasons) and afterwards would naturally take these vehicles home with them. These vehicles began the import invasion of the 1950's and 1960's in the US. Once these cars had arrived, the door was open for European manufacturers to come to (or come back to) the US and sell cars.
In this era, there was no Federal safety standard for motor vehicles. In fact, the individual states were responsible for this - and the standards could vary greatly from one state to the next. What was mandatory equipment in one state would not be required in a neighboring one; or in some cases, would be illegal in yet another! This was understandably confusing, and the Federal government was called in to formulate one single standard that a maker could build to that they could sell in all states.
Unfortunately, they did. While there were some valid safety concerns addressed by the government (lack of padded crash rolls, for example), the lighting issue was apparently addressed by both domestic lobbyists and what appears to have been neo-Luddites.
You see, when the servicemen brought back these European vehicles, their lighting systems were notably better than their US counterparts. While the Europeans had applied the lessons learned in war by producing what later became the Halogen-based H-series lighting standards, the American
manufacturers had gone back to what they'd been making before the war - sealed beam incandescent units - where the lens, reflector, and bulb were all replaced as a disposable unit. The American beam pattern tended to be much what it is today - narrow pattern, fuzzy cutoff, developed more for being seen by other motorists than for illuminating the road. The Europeans, lacking the vast electric networks in the USA, had developed lights that had a wide but bright pattern, a sharp vertical cutoff (to avoid blinding other motorists), and a more efficient illumination source; all of which combined to create lights for illuminating the road, not for notifying other drivers of your presence - that's what the little city pilot lights were for, after all.
The American makers, not wishing to lose their profits on selling the sealed beam units, lobbied the government when the standards were being set. In addition, persons came forth from the populace and
complained that the European lights were "too bright", "blinded other drivers on the road" (despite the sharp beam cutoffs), "glaring," "distracting," "dangerous" because the lenses weren't generally renewed every time the bulbs needed changing ("and we all know that we have to change the lenses every time for best results, right"), and a host of other complaints that didn't have all that much basis in fact.
These factors, and others, affected the government's decision on lighting systems, which were finally set as mandating equipment, rather than a performance standard. First, the specifications were set as follows:
*You could have either two large 7" headlights or four smaller 5.75" headlights.
Reasoning: "7" lights are brighter and cause more glare, therefore, you can only have two - and you should only need two to light up the road."
Fact: Wrong. Two 7" lights do not put out as much light as four 5.75 lights do.
*The headlights had to be round in shape, and of two specific types, which today are known as the 6024 and the 5006/5001. They must be a DOT approved assembly, and must be an incandescent sealed beam as the European bulb/lens/reflector system was not acceptable and "dangerous'.
Reasoning: See prior discussion.
Fact: Wrong again. Time has proven otherwise - and the Halogens aren't "dangerously bright".
DOT later modified its regulations to allow rectangular sealed beams - but left all the other restrictions intact, so still limited to two large, or four smaller units. In the 80's, when it was blatantly clear to everyone that the laws just didn't make sense, DOT finally gave up and went to a performance based standard (which is what it should have done in the first place) - so long as you conform to the light pattern standards, DOT no longer cares what you do. Halogen, HID, freeform or composite headlights, whatever - which allows for newer and more effective lighting schemes to be developed, while still maintaining some minimum requirements.
Bottom line, then:
At the time of manufacture, 7" outer lights with 5.75" inner lights was illegal for all US XJ's - thus, we got them with four 5.75" lights instead.
Currently, since 1990, reconverting your car to 7" outer lights is permitted. If the cars were still made today, they could be imported with the mixed 7"/5.75" setup.
The lenses make the difference - installing European H4 lenses is not legal, as they do not meet US DOT beam pattern requirements. However, installing the DOT-certified H4's, also known as HB4's, is perfectly legal. Likewise H1's (the high beams). Straight EU lenses illegal, DOT-approved counterparts legal.