They go at the bottom of the dash. You can just see it near/behind the shift knob on my '69. Same location as my '68 but I don't have a photo showing that one.
Formula, you sure about that ? I had a '68 Hemi 4-speed RR, 9/30/67 SPD, which would have had an Inland when new. It was swapped for a Hurst (the aftermarket unit) before I got it in the early '80s. It had the reverse indicator, wired in just like factory. I pulled its dash to restore it when I restored the car in the later '80s; nothing indicated it wasn't there from Day 1. All my other '68s were later SPDs or automatics, no help there...
The 2 I've seen were Hurst. I suspect that the light was moved at some point by a prior owner? Admittedly, I've only seen photographs, so I never was able to investigate further. I was thinking that maybe they were re-located up high for track purposes, so the driver could easily see when the trans was in reverse?