Thanks Hondaman for sharing pics of what must have been quite a job. We now can make visual comparisons.
The following might be interesting for CB500/550 riders, especially in Europe. Originally the CB500 model came with the D7ES plug. Before I began maintaining my CB500 myself, I had it serviced at least two times by an official Honda dealer. They have always fitted the D8ES-L sparkplug. According to p.83 of a French manual*, which was composed in close cooperation with Honda France, with the arrival of the K1 there the CB500s had the D8ES-L fitted. In Holland, after the break-in period, dealers also would fit the NGK D8ES-L sparkplug. When I asked why this was - the CB500 owner's manual specifies the D7ES type - the answer was that the D8ES-L was a better choice, seen the cruising speeds these bikes were operated at on the highway. I don't rule out that this preference for the D8ES-L had even been a directive by the importer, Honda Nederland.
About these cruising speeds, in my archives I have several riders reports and in all of them owners reported an average cruising speed of 130km (80 miles). On a CB500 that equals a 6500 RPM in top gear. Maybe importer and dealers, knowing this, wanted to be on the safe side, but there's more. Forsaid French CB500 Manual on p. 83 points at the particularité of the D8ES-L being autonettoyant which means selfcleaning and recommends them for the earlier CB500s all the same.
That last recommendation is my starting point for a bit of guesswork. The CB500 was launched in 1971. Now these bikes had been assembled some time before ofcourse and likewise had the Owner's Manuals been written long before the bikes were in the showrooms. Could it be that when the first CB500s were being assembled and the owner's manuals composed, the NGK D8ES-L sparkplug was not around yet?
Realise that in general these bikes in Europe have indeed been used differently than those in the US. This has to do with a difference in infrastructure. In the US it is quite common that when you leave home, you walk to a vehicle, even to buy a loaf of bread. It's an automatism. In Europe and certainly Holland the situation is different. Shops are nearby and to do your shopping, you just walk or ride your bicycle. Cars and motorcycles are reserved for longer distances. So in the US in general these bikes will see more shorter rides than ours do. BTW, the in this forum often called out 'poor' charging system which never has been reported as such in Europe, also indicates to that. This difference in usage could well explain the preference in Europe for the D8ES-L, also for the CB500/550, where the US sticked to the D7ES. Anyway, with this in mind, any of you can decide for him self what plug is best for his rides. If your CB500/550 sees over 5000 rpm often enough, you may consider the D8ES-L which for me has always been a superb plug.
* From that same French manual I learned that NGK had the letter 'S' for plugs that were thermo-elastique and multithermique.