Fascinating. This must be the original design of the Key Tronic foam & foil - no metal plate and square sliders. I've got a Key Tronic foam & foil from 1979 and the PCB is the same (round capacitive pads) but it has a metal plate and round sliders.
Notice how the foam pads on these disintegrate. It seems to always happen on the ones with brownish pads. When they changed the design in 1981 ("butterfly" shaped capacitive pads), they also changed the foam pads to a green material that
doesn't disintegrate. I have three of these keyboards, (1982, 1983 and 1989) and none has even a single damaged foam pad. So the bad reputation of disintegrated foam must be from the earlier design brown pads.
Key Tronic certainly improved their design with the post-81 "butterfly" version. Besides the pad improvement, the PCB much better constructed. It's beautiful. When I disassembled my 1989 keyboard, I expected to see late-80's cost cutting, but I almost gasped at the beauty of it. (probably one last bit of quality before the horrible 90's came along to ruin everything)
By the way, this is a Sanders 8170 keyboard.
Sanders 8170 1976.jpg
Sanders 8170.jpg
The earlier Sanders 720 (as seen in the header) used Micro Switch.
