Not to hijack the thread but your post got me motivated to take stock of my spark plugs, caps, and wiring. Original first generation NGK non-resistor spark plugs for the Honda CB750 and 550 (earliest plugs are believed to burn hotter than later replacements) and original Honda spark plug caps. I also bought vintage Packard tinned twisted copper ignition wire and NGK wire splicers (nearly impossible to find now).
Both bikes start so I am saving the re-wiring for another day 
Photo of the wiring for the CB750
Late Edit: I don't know why I bought 3 sets of spark plug caps???
Can't have too many sparkplug caps!

The 'early' NGK sparkplugs that worked so well for the 750 were the specially-designed D8ES-L that has a longer 'snout' inside the threads than the D8ES, and a little shorter than the D7ES. In measuring them, the D8ES-L length, if 'converted' to the difference between the D7 and D8 heatranges, is like a D7.4 (some say D7.6) sort of heatrange. It came out at Honda's request to help keep the CB750K1 plugs cleaner longer when ridden in mostly-slow commuter service, and when the 55 MPH speed limit hit in the USA, became an almost perfect all-around plug for the bike. The equivalent in ND is their X24ES-U, which runs slightly hotter than the D8 range.
The "change" in the plugs came in the late 1980s when the US DOT called NGK a "liar" very publicly, stating that (since Motorcraft couldn't make one) "no sparkplug can cover that many heat ranges" that gave the plugs the "S" ending to their partnumber - which in NGK-ese meant "Superwide heat range", covering from D7 to D9 range, as compared to their other plugs. Then the last letter became "A" instead, and the DOT here forced NGK to advertise their plugs as having narrower heatranges than they do - but, the D8 is still a bit too cold for in-town use (always was in the 750), while the D7 is a bit too hot in the 750 for touring at 70+ MPH. This re-designation was, and still is, purely a political event caused (in the USA) by people who are no longer employed in this country's government (and good riddance, IMHO...).
Meanwhile: the D8ES-L sparkplug, made expressly for the 750, was discontinued. When another Japanese engine needed a similar plug the DR8ES-L was brought out (around 1996) and can work almost as well, so not all is lost. If you have to use non-resistor plug caps, this one has a 2.5k ohm resistor (in the ones I have measured) inside, out-of-the-box. I've used them and they work OK, although my favorites are the ND X24ES-U or X22ES-U plugs - easier to find.