Working on that American muscle car back in the 60s, you didn't have a computer diagnostic port where the car told you what was wrong, if you thought the ignition might be an issue, you checked the plugs, the points, the distributor cap and all of the associated wiring. Maybe you changed the springs on the centrifugal advance to change the curve, hoping to get the best from your engine.
If all of your efforts failed, you took it to the shop where they rolled out the Sun or King distributor test machine and analyzed what was going on and put things right. They did what even a very capable DIY mechanic couldn't do without one, verify the advance curve, set the dwell angle very accurately and make sure everything stayed that way as the rpms increased. Nice machines, but it's been a long time since you could find one in your local garage or dealer service department. No problem today, unless you have one of those old cars, then what? Buy an old original if you can find one, but does it work? Why not buy new?
Sun is long gone as a manufacturer of test equipment, but King is still, or rather, again, in the business. The King Company, originally from Cleveland went out of business, but the rights to the name were purchased by Deke Williams, and the new King Electronics company in California, is building King distributor testers that do everything the originals did, and more, with just about any distributor or magneto. Forget trying to resurrect an original, these are brand new and guaranteed. Made in the USA, too. Pretty sweet.
UPDATE: Looks like they're out of business.
King Electronics out of business
has them for sale on eBay No longer available