Originally it was NGK D8ES-L. The D8ES is just 1/2 a step colder. I'm not sure whether it is the D8ES-L or the D8ES that became superseded by the D8EA. Maybe someone else knows.
The 1969-'71 plugs were D8E. Then Honda asked NGK and ND for something that would foul less while not adding the heat that the D7E did: the result was first the NGK D7ES, which was the D7E with a superior insulation seal to keep it from burning itself out a highway speeds. It was a great improvement for town-ridden 750s. Soon after, the D8E became the D8ES, too, with the "S" denoting "super wide heat range" on the NGK boxes.
In 1970, ND, who is the parent company of NGK and who made mostly auto sparkplugs, got into the fray with their own plugs. They had long sold the X24E, which was equivalent to the D8E, but with a thinner center electrode: we racers liked these at the time because they ran slightly hotter that the D8E, and cost 1/2 the price.
In 1971, at Honda's request, NGK introduced the D8ES-L, which was the D8ES with an extended tip to burn off deposits. It was a really good thing, and helped with almost all of the 750s. The hotter tip stayed cleaner longer, and the heat range was approximately like a fictitious D7.8ES might be: just a little hotter, but not too much.
In 1971, ND introduced the "Hot U" series, which I thought was just an advertising gimmick: they made U-shaped (like a C-channel) ground electrodes and put them on their plugs. Pundits said "it makes room for a bigger spark". Turned out, they and I were dead wrong: they had also "warmed up" the plugs by making all of the "Hot U" plugs with an even narrower center electrode and also extended it even more than the D8ES-L tips: the result was immediately recognizable if you slipped a set into your 750 or CB500. The midrange was stronger, the MPG better, and the plugs stayed cleaner: a true trifecta for this bike. We stocked both the NGK and the ND, but never could get enough of the ND after word got out. the ND number is X24ES-U, with a heatrange equivalent to a fictitious NGK D7.6ES (or -EA).
Then they introduced the X22ES-U, aimed squarely at the CB500. Same story: another success. This plug is about the same heat range as an imaginary NGK D7.25ES, and was perfect for all but fast, all-out touring in those engines: we installed the X24ES-U in the 500 for that faster and all-day touring service, with excellent results.
The ND is still available today, and it is the first choice in my bike. I regularly clean and re-use them: some of mine date back to 1982, and work perfectly still. I run the X24ES-U in all summer riding, touring, and high-speed chasing, and the X22ES-U in wintertime so the engine warms up a little faster in commuting use.
The NGK designations began as "D8E" or D7E", and there was a "D9E" and D10E" until 1971. Then they all became the "S" series, like D8ES, which meant "super range plug" or wide-heat-range plug. This just meant they would not burn out if subjected to overheating, and it was true: the D8E could be burned out in 100 MPH touring on the CB750 at the time (I burned them out regularly in both my K1 and my present K2), while the "S" plugs survived it. The D9E disappeared with the "S" series.
When the Dems dropped the national 55 MPH speed limit on the nation, the D8ES would not stay clean, even in 60 MPH touring: that's when the quest for something better made all the above items happen. The D7ES would burn valves if toured all day, and 12-hour day rides were not uncommon, just to get somewhere at 55 MPH. The X24ES-U solved all of that.
