Terry, this thread seems to obey Henry Kissinger's law of academic battles: "The battles are so bloody because the stakes are so small."
I suppose his law applies equally to aircraft trivia. Now knowing more than I have ever wanted to imagine about the subject, a few calls to accessory builders seems to indicate that 10%-15% of their magneto business involves dual-impulse systems, and those are tending to be small Continental engines. This is far more than we would have imagined, although far from being ubiquitous as well. Of course the original question was why the switch read R-L-BOTH, and that is obvious if you consider how the switch has to be engineered: L has to be next to BOTH, whether you put the label on top, bottom, or sideways.
Related question is why have two impulse couplings at all? Positives: engine MAY start a little easier (but that's a stretch). Negatives: More weight; more complexity; no performance advantage; decreased reliability in cruise; engine may start with one coupling damaged, leading to a gear-train crash in flight causing total engine failure (and this is the biggie, not to be ignored). Possible answer: may market better to aircraft mfr. or be a sales point to a customer, many of whom like to buy everything they can get to put on their airplane, whether it makes sense or not. At the time, engine manufacturers probably were told the couplers were bulletproof. However, numerous ADs in the late 90s regarding coupler wear and in-flight failures should inspire caution, and older couplers should be updated to the snap-ring design..
Regarding anti-seize, no Darrell, your message is informative, nothing to waste a bullet on. Thank you for your comments. I don't make a living as an aircraft technician, but I have removed a few seized plugs in racing engines, and aircraft, and my low-hassle method is to wrap a few rags around the plug, pour some liquid N2 on the rags to saturate them, and wait for the 'plink', usually less than a minute, then unscrew the plug with a leather-gloved hand or a plug wrench. A gallon of N2, ferried from the welding shop in a plastic water cooler, is plenty, and worth the trouble for a $4000 head, or an aircraft cylinder..safe and easy.
The WWII spark plug anti-seize was originally metallic, until the Pacific incident, then they switched to "anti-seize, spark plug, graphite and petrolatum". I'm looking for a can, has a MIL number. Good because it is non-metallic and burns off the plug tip, clearing a short, but only moderately effective. The Champion anti-seize is a thin-film graphite coating, safe but not too effective, as my plugs squeak and groan when they are removed after using the stuff. I prefer to use a thin coat of Bostick Never-Seez, the medium nickel-rich grade. I have used this stuff on Hastelloy C and Inconel X-750 tubing and Swagelock fittings carrying anhydrous HF gas at 500 degrees C, and it was, of all we tested, the only anti-seize which would work a t these extreme conditions without galling, allowing the fittings to be reused. To use it on aircraft plugs, wipe a thin coat on the threads, and wipe off as much as you can. It's flawless, as long as you torque to about 85% of recommended torque.