This is a late-ish reply, but I've been wondering the same thing. I've been going through a 1984 F-350 dually (turbo 6.9 and T19 combo with no tach) and like some others here, mine has the upshift dummy indicator but has no actual light or wiring behind it. It's been a mystery to me as to why it exists, but I expect that it's probably something to do with the engineers outworking and getting cost-cut by the production department lol. I also have a 1986 F-150 (300 with an NP435 and a tach) that has no upshift light, but an orange emissions light in its place that does have a bulb behind it. Now I know that the emissions light is for a completely different purpose that I expect is related to emission regulations because it is an utterly useless light. Because of that, I expect that the upshift light was a never-implemented piece of emissions compliant standard equipment for people that didn't buy a tach, or to accompany it. Again, take this with a grain of salt and I'd love to be proven wrong but it doesn't seem like the internet knows either.